Wallace is offering good advise, even if that doesn't turn out to be your problem. A while back my 68 would get an occasional bad skip. Then, it would correct itself. Turned out to be a wire to the starter. Where the starter wires run by the exhaust, the insulation had a very small burned spot --just enough to make it skip.
As I recently said on another thread, I have a friend who runs an auto electical shop, and he says that wiring is a common problem in a lot of restorations --especially ground wires.
Let us know what it turns out to be...