Just married a 240SX and need some help

Ok, so I just married a guy with a 1990 240SX. He loves the car, but unfortunately knows next to nothing about mechanics so he has done none of the work himself. Well I know a little bit, though extremely complicated jobs are outta my skill level.
But here's the problem, this car has definitely been abused. I have worked on it a little bit, but nothing serious. Just a few oil changes and a tune up when he joined the military as a welcome home present. So my knowledge of this car is very limited. I know that whoever designed it, and put the oil filter where they did should probably be beat, but such is life. It sat for almost a year with very little driving time, and now it has become our main form of transportation.
I just changed the oil on it the other day, and decided that next go around an engine flush was definitely in order. I looked it over and didn't see anything else that was suspiscious looking, so I called it good. Well two days later it began acting as though he was popping the clutch, only this is an auto transmission. We have noticed it only happens when the vehicle is still kinda cold. I'm just wondering if I should contribute this to the old age of the car or if this is an indicator of another problem????
He has been complaining that it no longer idles as high as it once did, but I don't know what the norm is for this vehicle. I know that it idles at about 1000 when it is in park but drops down to about 700 when you put it in drive. That seems like it is okay to me, but he keeps complaining that it isn't. Is it a problem, or is it just him being a hypo-car-driac which is pretty usual??
I don't know much about the specs of the car as I have yet to really warm up to it but I do know that it hasn't had a lot of after market modifications done to it. It only has a four cylinder engine and an auto transmission.
Thanks for any help you can give me.

sounds like you have a transmission that is thinking quite seriously about giving up the ghost. check the fluid level, consider a flush, and put in the thickest additive you can find. if that doesnt help, go on ebay and find a manual trans swap.

oh and you may have some sort of idle issue. when the car is warmed up it should idle at around 750-850 in park, and a bit lower in drive. theres so many different possible causes for a low idle that id need a lot more info in order to even guess.

otherwise awesome car. it only comes with a 4 banger so if you want more power, buy an exhaust powered hairdryer. light weight, good handling, 4 wheel disc brakes, your motor has about 140hp and 155 lbft of torque. its the slightly less powerful but still strong KA24E 12 valve motor. dont let anyone tell you this motor isnt worth keeping.

So you are saying that if anything it is idling to high? Though not by much. So we need to keep a close eye on the tranny. He just had a tranny flush about a year ago, and then it really started giving him problems he says. I would have stuck with regular drain and fills with a car this old and abused. I'll start looking into some other parts, though he seems to think that an auto transmission is better than a manual, so I'm not sure if he'll want to swap.
The motor seemed good to me, though an overhaul certainly couldn't hurt it. It's on a to do list, but I have to get my car out of the shop first.
Any other suggestions? Any tweaks to make it run a little smoother? Any preferred oil types/brands (this last time around I ran Castrol 5-30 High Mileage through it)? Is there a good kind of Spark plug and wire set for this car??? Oh yeah, and we live in Utah, so any cold weather maintenance tips would be nice too. If I am going to spend lots of time under the hood if this car, I might as well do it right the first time. Thanks!!!

all depends on how much money you wanna sink into it. if you have absolutely no oil leaks(be 100% sure) you can switch to amsoil or some other high end synthetic, but if you have even a smallish leak itll make it worse. i would use a high end conventional oil(like plain ol' castrol gtx) until the motor gets rebuilt. stick with OEM or NGK plug wires, and denso or NGK spark plugs. you dont need anything fancy.

tell your man that automatic transmissions suck balls. big hairy salty sweaty balls. lose the slushbox. it was made for people who drive altimas back and forth to their insurance desk job and dont have the mental capacity to actually drive a car. the only thing an automatic lets you do is ummm...drink coffee and get road head while driving.

keep your coolant condition up, keep a good battery in it, if you get snow i would invest in another set of rims and tires for the winter.
the only reason not to drive this car year round is if you dont want road salt to even touch it. i drove my old 240 around minnesota for a winter and it was a blast. never got stuck. even in snow that was higher than my floor pan.

check the idle air control switch, coolant temp sensor, mass airflow meter, check for vacuum leaks, check the oxygen sensor for switching (you need a good analog voltmeter for this car, one that reads very low voltage in real time. oxygen sensors switch between 0 and 1 volt usually, with variations in .01 volts). after that, just keep up maintanence and have fun driving.