HELP!!!! - 1997 Monte Carlo - PROBLEM

Okay heres what problem I am having, and what I have tried to fix it. I get an error code called "Multiple/Random Cylinder Misfire". The car will also randomly "fix" itself, usually while I am driving down the highway. The first four or five times it starts to correct itself, it will slip back into flashing again, but it will eventually go away. The final time it quits flashing, the service engine soon light will quit flashing for good (until next time), and stay on solid. It doesn't ever go off, it will just stay on solid untl the next time it has a problem, when it will start flashing again.

The car will run fine from 1 day to 2 months with no problem, but randomly, if I try to start the car, it will start but it acts really rough. It eats gas like there is no tomorrow (or bottom to my checking account.) It doesn't stall, but shakes horribly at lights w/o the brake firmly applied. At high RPM it runs pretty smooth, until i slow down, when it sometimes stumbles to keep going .

I had first suspected TBI, which is clean now, and have run SeaFoam through the PCV line through to the Intake Manifold and that hasn't fixed it. I have changed all of the plugs, wires, and coil packs. I checked the MAF sensor, and all sensors on the Air Intake hoses. The starter has been tested, and works fine. The Alternator is still putting out enough power. The PCV valve and Air Filter have been changed. All 6 injectors sound great, no offbeat sounds, no oversprays. they all have a clean ticking sound.

Taking off the oil fill cap, I did not see anything on the lifters, or any damage inside.

*** Also: The sound the exhaust makes is kind of a chug chug chug. If i put my hand close to the exhuast it blows the same way.

I drive 40 miles to work each way, which at the current MPG I am getting is like 19 dollars a day in gas. HELP!!

Any help would be greatly appreciated!!!!

TROUBLE CODES:

P0303
Cylinder 3
Misfire Detected

P0420
Catalyst Efficiency
Below Threshold
(Bank 1)

There are a few others, but I know they are because I unplugged sensors.

Thanks,
Streetchariot

I am not saying I know much about cars but the #3 code may be a spark plug wire misfiring.This was a problem I had on my 96 Monte Carlo z34 with the 3.4 engine and after I replaced the spark plug wire it eliminated that problem.It was shorting out down in the cylinder because these wire go into the head.Have you had the catalytic converter checked for being plugged up.That chug chug sound could be caused from this.Had a car that made a noise similar to that and it was the exhaust backing up and leaking around exhaust gasket.This could also cause other problems because it is messing up other things like the oxygen sensor ect.This is just past problems I have had and food for thought.Good Luck

the best way to sole this is to do this.

1 take the plug out of Cylinder 3 and move it to Cylinder 4

2 take the wire from Cylinder 3 and move it to Cylinder 2

"you dont have to pick those 2 Cylinders as long as you do not move the plug and the wire to the same one."

3 clear the codes.

When the light comes back on see what Cylinder the missfier has moved too and you got you awnser.

also the cat code is from Cylinder 3 not fiering. you are dumping raw fule into the system "this is bad!" when the light is flashing like that it is telling you that you may hurt that cat.


EDIT* and if it has a coil over...or more then one coil move the coil for that cylinder too