Make sure you have a decent flow of fuel right into the carbie and that it doesn't weaken out after 10 minutes. (if it does, then you have a partial block,) If that's ok check your condensor in the distributor.
It could be the coil breaking down under heat as it gets hot, if it's real hot too touch the coil is on the way out.I do feel like it's worse as the tractor warms up. Could that be the coil???