93 eagle talon.. starts fine, then sputters?

the car starts fine and idles around 2000rpms for about 20 to 30 seconds. Then the rpms drop to around 500 to 700 and then the idle becomes very rough and sputters( sounds kinda like its running out of gas). Then when i accelerate, it seems to have loss alot of the horsepower, very slow acceleration. Plus while driving, the rpms are about 500 higher than it was before. Ive checked the air intake and coolent system for holes, found none. Im still getting sparks from the spark plugs, so im starting to run out of ideas. any help is greatly appreciated