I think I have solved the problem of hard starts on damp mornings. My car is a 1993 940 8-valve wagon with Rex/Regina ignition. Car has 210K miles on it. I had intermittent difficult starting on most damp mornings. Once the car started in morning, it would run fine and start the rest of the time until the next damp morning. Not all damp mornings, but most. That's what made diagnosing so difficult. I read a lot and decided to replace the RPM (crank position) sensor. The wire casing on the original looked very old and dry. I am pretty sure it was the original sensor, so I had no qualms about replacing it. I also took the coil off the power stage and the contacts there were pretty green with corrosion. I cleaned the corrosion on both parts and packet the contacts with dielectric grease. That's all I did besides making sure the ignition wires looked good and contacts were dry and clean (they were). So far it's been a few weeks of very damp weather and so far so good. I am still not 100% sure I solved it, but on mornings when I knew it would normally have problems starting, it started right up. Also, it seems it does not crank for as long before catching now, even in dry weather. I think the real culprit was the corroded contacts on the coil, but I think giving it a new RPM sensor at 210K miles is probably not a total waste.

Thanks to all who responded with good suggestions!