Did you check the 80 amp fuse in the relay box? It is a bar fuse, just a strip of metal. Screws on either side could be loose, or it may have a crack that you can't see unless you flex it a little bit.
And I would say a GP could go bad but still work some of the time. If the little coil inside that gets hot is broken vibration from the engine may make it make contact on the next start. One plug is not going to make for real hard starting anyway. I didn't notice mine were bad till 2 went, and then it would start OK, just idle pretty rough till all 5 were firing. When three went which included the number one plug and no light came on it was next to impossible to start below 45.