i have a bryant plus 80 furnace. which seems to function fine, except at start up. When the thermostat calls for heat a evac blower turns on to clear the chamber, runs for a few seconds the the ignintor starts to glow the gas valve opens and the burner lights but the goes out. this process has happened as many as 3 times before the burner stays lit. I have been told its the gas valve, the plentum is cracked and is getting a back draft, the burners are dirty and just need to be cleaned. I'm pretty mechanical and can do the repairs, but i'm not a hvac guy so i'm not sure what is going on, anyone have a clue what itr might be??