Two things to check--one easy to fix, the other not so easy. First, the vacuum advance on the distributor. The harder one is to check the timing chain. Put a wrench on the crank bolt and with the distributor cap off, check to see if the rotor moves in synch with the movement at the crank. If there's any slop, you've got a project to strip down the front of the engine to change the timing chain and gear. Once done, though, you'll be good as new.
Cheers,
Roger