I have an auto transmission 1988 Maxima SE with 112k miles. About a month or two ago, my car started running kinda strange. It no longer had the pickup it used to have. The car shakes a lot. A lot. Especially when I turn the A/C on, it shakes pretty badly and when it's idling, it will momentarily dip down to 500 RPM. Without the A/C on, it fluctuates between roughly 700 and 900 RPM.

When starting from a cold start, my car will run perfectly fine for the first minute or two, or when it warms up to a sufficient temperature, at which point it starts running like crap again. At first, my dad thought it might be a fuel injector problem. He had me switch over from the Chevron 87 octane I usually fill up with to Chevron 91 octane to clear it up. Two tanks and there was no change.

My dad then thought it was the exhaust. He had the catalytic converter and O2 sensor replaced. No change.

I got an oil change, and it helped slightly, but on the whole there was no change. After the oil change, I was told that my transmission had a slight leak.

My dad was "convinced" that the shaking is due to the transmission slipping. Recently, he did a drain and refill of the transmission, and poured in some Lucas Transmission Fix. This was two weeks ago, and there is still no change.

I have no idea what's going on here. Here is some more information that may or may not help. I have exhaust installed. When I'm driving at a constant speed, my car does not hold a solid tone. It's kinda hard to explain through text, but when I'm driving it sounds like "brr-brr-brr-brr-brr-brr-brr-brrrr-brrr," like I said it's hard to explain through text, that's the best I can do. It's like, when you're going, and you have that revving sound going up in pitch in a solid "motion" I guess you could say, instead of that, on my car it's not a consistent sound.

I hope you can help. This is making me really angry. I feel so slow.

 

Check Ecu for codes. Sounds like the emissions systems or the TPS. But check for codes to pinpoint the problem. Its under the passenger seat.
Go to town!

 

Well I was having RPM fun recently and I decided to do that Air Regulator Fix in the stickies. Fixed my problems and I can't see it hurting anything to try it. Also try checking that your Cylinder Head Temperture Sensor is workings correctly. But along with what Drjmaxx88 said, check the codes on your ECU, they can tell you a lot of problems if you have them.