Check Engine Light sometimes comes on

My check engine light ('92 Galant) comes on sometimes after the car has been running for a few minutes. The frequency of this happening has been increasing (a few months ago, it might have come on once every couple weeks... now it comes on about half the time I'm driving the car). The car does not drive differently when the light is on as opposed to off. I found this website (http://www.troublecodes.net/mitsu/) describing how to check the problems, but I'm not sure if it will work or not, since the Check Engine Light doesn't come on until the car's been running for a few minutes, if it comes on at all (and the website talks about checking without starting the engine). Also, my engine revs high occasionally as well, though I have not noticed a relationship between it revving high and the light coming on... however, the car idles at about 1200 (normal is about 700), and has no problem speeding up to 42 mph without me ever putting the gas pedal on (I've stopped the car, put it in neutral after this happened, and the tach read a constant 2300 rpm... this happened yesterday... I dunno if it's a one-time thing or not, but if these all sound like symptoms of one problem, please let me know).

I'd appreciate any info.

i had the same problem on my 94 galant, and i find out that you have to adjust the throttle body sensor or just change it to a new on and then adjust. sounds like your sensor is going bad and you have to keep adjusting that.

Ps: you only able to adjust sensor with the snap-on scanner or dealer