92 ford ranger backfiring (new problem)

Posted thread months ago and finally got a chance to check timing. I can correct backfiring by adjusting the timing but the truck runs rough. A new problem has now started. Truck runs great until engine light comes on and stays on. Then it runs rough. But, just as soon as the light goes off, it runs as it should. I just noticed interior lights dimming and brightening as engine light goes on and off. Also I noticed the exhaust is getting extremely hot and the floor board becomes too hot to touch. Please help. Any advice? Tune up has already been done.