Start by checking all your ignition bits and bobs. (leads, plugs, points)
Timing could be a factor, maybe a slight leak from your vacuum advance hose?
Maybe the dizzy itself is a little gummed up, hard to say, start checking everything from the beginning and see how you go......
*Edit* Could be something with the jetting in the carby too, overactive accel pump, partially blocked jet, who knows, start with the easy stuff though.....