hucklebery

OK. I have an intermitent (-1 sp) check engine light. I start my truck and after about 3 minutes of driving the light flickers on and off. If the truck idles for a few minutes the light will come on and stay on. Turn the ignition off and the light goes off and will not come back on until the engine is cold again. Seems to happen more in colder weather but I live in southern GA so not that cold. Are there any fluid level sensors wired into the check engine light? That is my first guess of the problem. Dont know. This is a 1990 F-250 gas 4X4 with the 351. After market shift kit. After market dual oil filter system. I bought it used about a year ago. Had the check engine light come on after driving it hard in mud and such twice in 10 months. Didnt think much of it because the light would go out after turning off and then on. The flickering and constant light started about two months ago.