Check the point gap and distributor shaft for side play. That can cause strange problems, problems at different RPM, temperature, etc. Check the centrifugal advance. It should turn a few degrees and spring back freely when released.
Is the thermostat in and working?
Have the valves been set lately?
Has the air filter been serviced, including the wire mesh inside the canister?
Surging is commonly caused by a too lean fuel mixture, so a vacuum leak can cause it.
Try a partial choke, see if that improves it. If so, look more toward mixture/vacuum leak than governor. The idle mix sets backward of modern carbs, turning the screw out leans it, in richens it. to set the high speed, turn the screw in a turn, quickly open the throttle. It should falter. If not, turn the screw in until it hesitates on sudden throttle opening. Then start backing the screw out 1/4 turn at a time until it will take throttle without hesitation. A single puff of black smoke is desirable.
Look at the governor linkage from the gov arm to the carb. If the ball/sockets are worn sloppy, they need to be replaced. Mcmaster Carr has them. Any other wear in that linkage, loose bracket on the carb shaft, loose throttle plate shaft can cause idle problems.