I have a 91 SE 5spd I bought and now has 142k on it. The car will sometimes idle back and forth at 800 and raise to 1200 RPMs ever second, then will find its way back to normal. I get 17 mpg on the highway when I recently calculated mileage, and the car hesitates slightly under 2500 RPM. I also can not seem to find the right position for the gas pedal to maintain a certain speed, always slows down or speeds up. Anyone know what the problem could be?

 

Put fuel injector cleaner. If it still continues, you might want to check your plugs and injectors. Maybe they are just a bit dirty and you don't have to replace them. Although in a near future it wouldn't be a bad thing to do.