Random CEL

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Hey guys, I got a friend who just picked up a turbo mr2, anyway he picked it up and everything seemed cool, bought it for about 6, test drove it blah blah blah. Recently his CEL has been coming on a lot lately, but at random intervals of say 10-20 minutes? and shuts off. Nothing seems to be really wrong with the car, the car pulls just as fine with the light on or off, no weird knocks, nothing. I