97 528i stumbles at about 2000-2500 rpm, an also misfires...

run the fault codes first to find the faulty sensor

Need to be more specific here. Runs rough...like misfiring? or vibrating excessively at about 1500, and again about 3000 rpm? If it's the latter, chances are your engine's balance shaft is not timed correctly. This causes a heavy high-frequency vibration/buzz throughout the car, at the aforementioned rpm ranges. If it's more like a misfire/stumble, look into your ignition system, or possibly a sticking EGR valve. (This will cause a mild surge/bucking under light accel, peaking between 45-50mph. Good luck!

An ignition misfire feels like the car is stumbling. Get it up to about 55mph and ease into acceleration, if it stumbles you might want to get a tune-up. If it stumbles at idle and gets better with acceleration, you might be dealing with a fuel delivery misfire(clogged fuel injector or fuel filter)

Problem sometimes is with cheap parts they are bad.Good wires,good plugs,good coils.

Has it ever been tune up,

Based on what you described, I would say something is blocking oil from reaching the camshafts when the issue happens.
If the lifters run dry the valves won't open.

There are a few conditions that will not set a code. For example, stumbles are often caused by ignition coil problems. If the problem persists at a certain RPM, could be an O2 sensor problem in closed loop operation not lasting long enough to set a code, or a fuel regulator or injector problem. I would get a good code reader and check readings from all sensors in the circuit (including misfires) to see if you can pin down the problem.

The charging system is doing its job It seems to have an electrical short which is draining the batt. If u have the know how to ck & repair electrical shorts then go after it, if not, take it in.

fuel filter couldn't be removed huh !!! hmmm i think there would be no better place to start with than the fuel filter then onto the air filter etc ..but replace the fuel filter for sure

iwould say try some octane booster sounds like poor quality fuel p0300 is a multiple misfire so start with that and i believe vettes are supposed to run 89 or better octane