Car idles low and sometimes stalls

I have a 94 Eclipse GS 2.0 n/t 5-speed. I had a mechanic replace the timing belt tensioner adjuster, it was bad (sucked too because I had to change my water pump a couple weeks before). I also replaced a motor mount insert on the upper-left motor mount the day before the adjuster.
Car was running nicely when I got the car back. But when night came, my car was having starting problems. The next morning I checked the battery and the positive terminal end was corroded (post was fine) and the nut on corroded bolt would not come off all the way too. So I left the battery unplugged for a long while, while I rode my bike to the store to get a new brass terminal end. After I replaced the terminal end, my car started a little rough, but idled normally at 800 rpm approx.
As I was driving later that night, my rpm would dip down to 500 when I put the car into neutral, coming to a stop (like a stop light), and it's still doing it (this is day 2 now. I've driven the car less than 20 miles though). Sometimes the rpm would just keep on falling and stall, so I would have to do the "press on the gas trick" to keep the engine running at stoplights. Also a few times, but not as often, my car would just cut off while I was driving. The first time my car stalled while I was driving, the check engine light came on right before the car cutoff. I Pulled over the side of the road, look under the hood, seemed fine. When I started the car again, the check engine light didn't come on, and it hasn't come on again since. But my car still cuts-off while I'm driving sometimes, and I have to wait a few minutes before it starts up again, other wise my car doesn't start while cranking it. It's weird though, because my car idles perfectly at 800 rpm while my car is not moving (like when my car is idling while parked). The rpm only dip when I'm driving, and I go to neutral and coast to a stop. If my car doesn't stall when I pull up to the stoplight, and the light is taking forever to change (more than 15 seconds), my idle will eventually climb back to 800 rpm. Any ideas on what might be causing it? Also, do 1g eclipses record the fault codes, or does the scan only work when the check engine light is on?
Also, i did add a Chevron complete fuel system cleaner additive and a Risolone high viscosity oil treatment additive to my car before the problem occurred (I'm realizing 10w-30 is too thin for my 166,000 mile engine. I'm gonna go 10w-40 next oil change). Not sure if that wold do something since I reset my ECU a few hours after I added them. I also just replaced my PCV valve just in case, but haven't test the car out yet (old one was working so I figure it's not that). Spark plugs changed last week, wires almost 2 years ago. New throttle body and throttle position sensor almost 1.5 years ago. New fuel filter 1 year ago. I washed out my k&n air filter last week, and it may have not dried all the way, but I did let it dry out for a while before I drove the car.
Your guy's input is always appreciated. Tell you what though, I spent so much money keeping this car running, I'm starting to nickel and dime it now. I love my car, but come on! Getting deja vu, thinking about my old R32 Nissan Skyline.

Unfortunately, that seems to be the biggest problem with these cars. They are money pits and things always seem to break... To me it sounds like you covered everything pretty well to troubleshoot the problem. What was your spark plug gap set to when you replaced them? I'm wondering though if the problem could be with your coil pack. I have seen that happen before with very similar symptoms...

It could also be a vacuum leak somewhere. Check all your hoses. Listen under the hood when the engine is on and try to listen for a sucking or blowing noise. If you're not sure, you can pull off the hose, blow through the oneside while the other side is capped off. If air still goes through the hose, there's a hole. Has happened to me quite a bit. Just a thought.

My spark plugs are gapped at 0.044 inch. When I pulled the old ones out, their gap was 0.05, so that's what the car was use to running. Car was also slugglish like that. I hoping it's just my old car is taking a while to adapt to the new changes to my ride after the ECU reset, especially with the additives being put it right before the reset. I hope it's not an expensive part. Are coils easy to replace? I thought I heard some sucking noises under my head, I should check that out for vacuum leaks. You guys think the 02 sensor could be going bad? the 1gb only has one 02 sensor right?
You said it blackcrow: Money Pit :runaround: Engine runs great, very smooth, but everything around it has been crumbling apart slowly. I also sure do love that when I cruise around town, I rarely see a 1g Eclipse. I would hate to have a Honda, everyone and their sisters got one.

Update: Car is running a little better. But the problem is still acting up, just not as bad (I basically mean my car doesn't stall as much, but it's still bogging down and occassionally stalls). Not sure if the new PCV valve is what helped.
It sounds like I can hear a hissing sound behind the throttlebody, towards the firewall. I'm check it out later. Also, the rpm dramatically lowering problem while coasting, seems to happen when the car is warmed up. While the engine is semi cold (I usually warm up my car for a couple minutes before I drive off), and I put the car into neutral rolling up to a stop sign, the idle seems to keep steady (no rpm dipping). When the engine is at normal operating temperature, is when it seems to bog down more. Also, when I start the car, the radiator/coolant and battery light remain on. The lights just dim down about half brightness and stay on. Then when I start driving the car, the lights turn off (I wonder if they are related).

Coil packs are easy to replace. And Dave is right, I completely overlooked the possibility of vacuum leaks. I would do a leak down test. Similar to the way we would do it on a turbo cars... Build an adapter to go on in place of your air filter and connect it to a low pressure air compressor. While the car is off of course, then give it some pressure and listen for leaks and see if you have leaks in the throttle body, intake manifold, vacuum lines, etc... Also the PCV valve was a good call to replace if you have higher mileage on the car. Spark plug gap should be good also...

So my car wouldn't start the other day. Battery wouldn't hold a charge. Turn out the alternator crapped out. Replaced it, and my idling problem went away too. I guess my car receiving a low voltage was the problem. Or also with the PCV valve...