crank,
I haven't had the joy just yet to have my 77B on the road yet. but, I have seen alot of others saying that the carb. can get too hot after the engine is running cause simmilar problems. Do you have the heat shield behind your carb? this could be a possible solution. Check out the other threads, I know I have seen similar cases to yours plenty on this site. Hope this helps, cheers!
joel
1976 MGB with a 1977 engine