This can be a couple of things, it can be a carb cleaning or overhaul. It could also be teh breaker plate inside the distributor is worn out and you can't keep a good dwell/timing on the car. The vacuum advance could also be worn out in the distributor. To check look at the pin inside the distributor where the vac advance hooks up. And try moving the breaker plate back and forth with a screwdriver. It will move forward back but not side to side.