Engine Check Light On-

My car is 10 years old and currently has a mileage of around 133k (it's old, I know). My engine check light comes on periodically (has started for more than two years now), but it usually lasts for only a day or so, and then disappears. One mechanic said nothing was wrong when last checked, but the engine light had already disappeared by then. Every time the light comes on, I feel that there is something wrong with the transmission b/c the acceleration becomes much slower than normal. When the light goes off, everything appears fine. Just today, I went on the highway and noticed the light came on once again; I was going at approximately 55 mph, and the transmission gauge was reaching b/w 3,000-3,500 rpms (at one point, it hit 4,000 rpms)! I knew something was wrong, and I couldn't go on the local road, so I stayed at the slow lane driving at most 55 mph. After getting off the highway and stopping at a traffic light, I proceeded to accelerate and almost immediately, the sensor jumped to 2,000 rpms rapidly. I did not slam on the gas pedal or anything but found this very odd. I recently purchased a new car due to the previous car's transmission problems. I really do not want to go through the expense of a new transmission again. Would appreciate any assistance. Thanks.