Possible ignition problem?

Hey guys I have a 340 and I'm thinkin it's having ignition problems because when it gets to operating temp. (180 deg.) it begins to cut out while cruising and cut out when you begin to lightly hit the throttle. Now it won't stumble when you floor it from a dead stop or when you run it hard. Only when cruising and at light take offs. The engine has a new 750 double pumper carb, engine timed at 34 degrees, and has the 60405 lunati cam with 10 inches of vacuum at 1000 rpm. This is about to drive me nuts so any info will be muchly appreciated thanks!