If the burner works correctly at first then one can assume some propane is leaking out of the air port instead of going up to the burner. When the propane leak is sufficient, it catches fire. So, you may have too much propane coming out of the orifice (jet), too much air mixing (very likely), or a leak (also likely). So, check for a loose fitting or broken fitting allowing propane to leak out slowly. If you don't find a leak, then reduce the amount of air that mixes with the propane. That should discourage any leakage from the mixing point since that leakage is what is igniting. Good luck.