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Hmmm...
How about some more details? Is this a fuel injection engine? I think it is, and the problem may be different if it's FI or carburated.
I'd first check the exhaust, as that will cause the symptoms you describe. If it's partially obstructed, it will run OK (but not great) under a light load, but the blockage will cause excessive backpressure at higher PRM's.
I'd shake the muffler and the catalytic converter just to see if either one rattles. This would definitely indicate a problem. Otherwise, try to locate a Cat bypass 'test pipe' that will replace your catalytic converter and let you drive the car to see if it performs properly without that component. You can get by just running an open exhaust as long as you don't do it for too long.
I'd suspect the Cat first, muffler second. You might not notice a change in the sound of the exhaust if the Cat is bad, but a bad muffler should be obvious just by the sound.
The fuel management system is the next thing I'd look at. Fuel injection uses a series of sensors and a computer to control the fuel flow, while carburated cars from the mid-1980's might or might not have a computer.
Your description sounds like the engine is running rich - too much fuel at higher RPM's. Pull the spark plugs and inspect them. Virtually any car repair guide will tell you what the various colors and accumulations on the plugs mean, but dry black soot on the plugs usually indicates 'way too much gas. Wet, geasy black gook is oil, which you don't want to see. Try the smell test; if it smells like raw gas, that's what it is. :-)
You may want to get (beg, borrow, steal, buy) a code reader if you need one to determine what error codes the computer may be trying to tell you. I'm not familiar with your car's system, but generally the 1980's cars used the OBD-II code system. A reader for OBD-II costs about $39 at www.harborfreight.com.
Find out what system - if any - you car uses and read any codes in the computer. Some GM cars would flash the 'check engine' light when you jumped certain computer terminals with a paper clip. I still keep a paper clip in my tool box! ;-)
Another problem you may run into - although it would cause the car to stumble and stall at low speeds, not high speeds - is the fuel pressure regulator. Again, it's not a likely suspect, but something to keep in mind.
*Check your fuel filter!* a clogged fuel filter will prevent higher flow rates of gasoline, so this my be your quickest and cheapest fix. If you can't remember when you last changed it, it probably needs replacing anyway.
I'm sure there are many others here more familiar with your particular model, so you should be getting a whole lot of help real soon.
Good Luck! Steve, Ridgecrest, CA. (Just southwest of Death Valley)
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