Mitsubishi Repair: mitsubishi problem, vacuum leak, random times

Question
Hello. I have a 2001 eclipse rs. I've got a problem that I have no idea how to fix or what it is.
At certain speeds, the car starts buttering, and the rpm on the tachometer is jumping up and down. It happens at random times, i.e. the car drives fine at one moment, and starts malfunctioning a moment later, then drives ok again. It mostly happens @ 4th and 5th gears, but sometimes also on 2nd. One of the suggestions I got once was a fuel injection problem. Any other possible solutions?

Answer
Leo:Why not get your car coded, in that you get your computer of your car tooked up to an inshop computer. Any problems the computer is picking up, it kicks it back out at you. If you got an AutoZone close by they would do this coding for free. The computer of the car controls everything in that car, with the help of a lot of different sensors, through out the engine and transmission. Being it comes and goes it could be a vacuum leak too. But those are very hard to find, so you would need a shop to find such a problem. Still I would try the coding to see what the computer might tell you. God bless: Linda