97 SLS 4.6L 98k "jerking" problem

First of all, thanks for looking, I have no idea where to look, and after spending over $1200 on tires brakes and an oil change, I don't have a ton of cash for diagnostics. Car was running just fine before the previously mentioned $1200, and this problem popped up suddenly. When it's cold, at a stop (in gear) about every 9 or 10 seconds it seems like there's a "drop", like it loses power for a second? This problem gets less and less pronounced as the car warms up, but then when accelerating or cruising around 12-1800 RPM in 3rd or 4th gear, the car hesitates and jerks. If I give it gas at this point, it backfires, then seems to pick up better. Anything beyond 2000 RPM and it seems to be fine.

EDIT: Forgot to add, I've not recieved any check engine lights (it works at startup) nor does the dash display show anything regarding a problem.

Not sure if these are related, but they might help explain my problem:

I'm not sure, but it seems to want to stay in 3rd gear at 60 MPH, or it's in 4th and the RPM's stay at 2000 for way too long.

Also, the fuel gauge hasn't worked properly for some time, it's correct until there's only about 1/3rd of a tank left, then it will creep back up to 1/2 tank.

I've checked the transmission fluid, all is well, bright pink/red and no burning smell. I suspect it's either a fuel, exhaust or sensor problem. Like I said, it happened SUDDENLY, as far as I can tell, anyways. It is pretty consistant, and does not seem to be getting any worse. Any help or places to start would be greatly appreciated!