The car is my 1980 245DL B21F K-jet M46 with just under 301k miles. This has been going on for almost a year, here's a summary of the facts/observations:

1) If it's raining or has been raining/snowing, the car will crank and crank but will not start or even give a hint of combustion occurring at all.
2) The longer the car sits, the less likely it is to start when damp.
3) The car starts and runs perfectly when dry
4) If the car was driven recently (within five or so days) and it's damp, it will start and run fine, then after a mile or so will misfire and then lose all engine power - there is no electrical shut down, just stops responding to the gas pedal and eventually stalls. In this case, it may restart and repeat the cycle after a lot of cranking.
5) When attempting to start, I hear the main fuel pump for a second each time when the key goes to run position from start position.
6) After a lot of cranking (when damp), there's sometimes eventually a backfire
6) The ignition system - coil, plug wires, plugs, cap, rotor - were all replaced with no change. Wiring connections at the coil were fixed too. Main engine harness was replaced with an improved OEM harness a while back. Recent O2 sensor.

I'm guessing something intermittent with the supply of fuel, but since I hear the pump after cranking it seems like it's downstream of the pump. Any ideas? Thanks for any clues or diagnostic suggestions -

Zack
1980 245DL M46 300k
Sold: 85 744TD M46 251k, 88 745T M46 249k, 89 245DL M47 149k