Black widow run problems
Sounds like it is overheating and dieing out, a common problem. Try setting it just slightly richer and it will run rich untill about 1/3 of the tank is used up, then will lean out and run great for the last 2/3 of the tank.
I also like to use the 5 fin Texaco heads, they seem to give a more consistant run for long duration runs (in my sport rc planes).
Just a thought...
AJC