I have a bryant from 1991. When I turn up heat the fan starts then ignitor starts, lights pilot and will run like that for about 5mins then shuts down and won't try to relight until I reset the power. When the pilot lights the flame is kind of weak. The blower is blowing it hard and it will occasionally go out and the ignitior relights it. I've found that if I bang the combustion box it will make the flame get bigger, I'll hear a click from the gas valve and the flames light and it runs and shuts down like normal. A couple times it even started by itself after I initially got it started, but every time I leave, the next day its like it hasn't run at all. Any ideas anyone?

The model number is 398AAZ036080 series A