I have noticed for a little while that when my car is at about 3,000 rpm, and i push the pedal to the floor, it will accelerate and then the rpm's go down a little bit, then back up. Also the engine light has been turning on occasionally and then turning back off sometimes a day later, or just sometimes a few hours later. I took it to get worked on and they ran a test on it and it brought up a code for something, but they couldnt figure out what the code was for. Has anyone had this same problem? What should i do about this?