94 corsica jerking on highway speed! Help me

I have 94 corsica 3.1 V6 140k
The car starts jerking (engine shaking really fast) on above 55 mph. Its really annoying as the whole car starts on a rythamic jerk. If I press the accelerator really hard to the maximum the shaking stops and the car goes fine. Also the car only jerks when i press on accelerator, only when about 60 to 50 miles an hour and above.
Any help will be appreciated, thank you.

Its probably the spark plugs. I too have a 94 chevy corsica and it has done the SAME thing to me. Each time it has been the spark plugs that were fried. Even if the front three spark plugs look good.... check the back three (the ones that are REALLY hard to get to).

could also be plug leads. I just went thru this with my 94 corsica after much pain and money paid to the "experts ". More often than not, the problem will be in your high tension system.
good luck

could also be plug leads. I just went thru this with my 94 corsica after much pain and money paid to the "experts ". More often than not, the problem will be in your high tension system.
good luck

Is your service engine soon light on, and if so, what is the code that is set? If you don't have the service engine light, The first thing I would do is a tune up. Change the plugs, wires, air and fuel filters, etc. If that does not cure the problem, have the coil packs tested by AutoZone, and replace if bad. If you still have the problem, Run some fuel injecter cleaner in your gas, identify and clean your EGR, and the IAC (idle air control valve). Beyond that, you may have to take it to a shop and get an estimate.