The catalytic convertor may be clogged.  Take it to a mechanic and have them test the emissions at the tailpipe, or look under the vehicle and see if the catalytic convertor glows red after driving at highway speeds or for extended periods of time.

Have you checked throttle cable, I had that replaced when it broke and the car then had fullpower.

try a fuel filter

the question is very vague, need more details of wat is happening, but, check your throttle cable, also check your cat, fuel filter,fuel line, is the clutch good ? worst case, might be a head gasket causing it to loose compression.

cheak one of the fuel injetors it maybe cloged

check for air leaks around the intake boot

like everyone else is syin it could be quite a few things, dont guess just take it to your garage for a check up.

i agree alot of times in these older cars it can be a problem with vacuum and its a pain in the ass to trace however take 15-20 minutes follow the vacuum lines if you see one unplugged and an obvious place to put it try that first then if not replace the fuel filter it will never hurt to replace a filter of any kind and its easy if still no good try the shop

My guess would be a vacuum leak as well. You can check for vacuum leaks by spraying carb cleaner around possable leaky areas ie around the intake boot and other connections. If the idle jumps you knnw you have a leak in that area.

also check the gap on your spark plugs many shops say they are pre gapped but it is no always the right gap.

also clean out your idle control valve with the carb cleaner.

If no of this helps come on over to e30tech.com tell them bottlecape30 sent you.

I had a similar problem with a 240SX when i changed the spark plugs and knocked into the distributor cap knocking one of the plugs out. Which cause the fourth pistion from firing. My question to you is does ur exhaust ping or pop when idle or accelerating?

I had a similar problem with my 320 - It was ok on a cold start but one walm lacked power. I took it to the local dealer and diags showed it had a faulty crank shaft sensor. It cost about £30 but it was well worth it. As other's have said, get it on a diagnostics machine, so many engine faults are logged there before you notice a problem...