Could have bad condensor or points, coil etc etc. If you have some spares I'd swap those out. Check ign wires, easy to cross 3 & 4. Vacuum leak will make you crazy also. If you have some B-12 Chemtool spray, it's great for starting help. Keep at it you'll find the culprit. I had one of these same frustrating events, took me three weeks to find the disolved paper fuel filter living in my carburetor.
Best of luck
