A VERY long time ago, I heard of a guy with a similar problem. Of course, I may be mistaken...because this happened 25+ years ago!

If I can recall correctly, I believe he discovered he had a bad distributor cap. It was cracked and would allow moisture to slowly build up inside the cap. Moisture would cause cross firing, etc. When the engine was warmed up, the heat would dry the cap and the engine would run fine.

I would examine the distributor and distributor cap very closely.