Without more info, I would offer the following possibilities: Sediment in the system such as in the gas tank, fuel pump and / or carb. Sometimes, fuel can be drawn from a tank with fairly heavy sediment without causing noticeable problems UNTIL - during driving - fuel is added during the drive. This ''stirs up'' the sediment in the bottom of the tank which is then drawn into the pump & beyond. You may also have a vacuum leak somewhere. Other possible causes may be related to the ignition coil, condenser ( in the distributor ). the list goes on.