Engine Light & Rough Shifting

I have a Chevrolet Silverado 1994 model, 67,000 miles, and 350v8. My truck runs fine whenever the engine light is off, but every once in a while it will come on. When this happens, the transmission shifts hard out of park, reverse, drive, etc. i lose overdrive, and it shifts out of first gear pretty rough. all i have to do is turn the key off and restart it and it runs fine. i may have to restart about 3 times. also, this may be a different problem, but the truck idles funny when this happens.it is not the transmission b/c if this was the problem, it would be shifting hard every time. this is not the case.