You missed bashby's point completely. If you are getting good spark to the cap, then the pickup coil is working. If the pickup coil were not working you would have no spark even to the cap.

Since you have good spark to the cap, the problem must be "downstream" from there. There are several possibilities (not listed in any particular order)

1) Cracked distributor cap - the cap could have a hairline crack. With the cap off of the distributor, it may look fine. Put the cap back on the distributor and every spark runs down the crack to the distributor body instead of to the plugs.

2) Cracked or otherwise bad rotor - again, you see good spark at the cap with the cap off. Put the cap back on and the spark runs down the cracked rotor to the distributor shaft and not to the plugs.

3) Plug wires - bad insulation or burned up core. Replace.

4) Fouled plugs

Then there is a very unlikely problem - bad timing chain. I had a friend with a 302 that suddenly started running weak. Ran that way for a week or so, then stopped running completely. Had spark, gas, etc., just would not fire. After checking a lot of things, finally found that his timing chain had stretched. It skipped one tooth after which the engine ran weak. Then it skipped another tooth and would not run at all. Since your problem seems to be related to the rain, I would guess this is not your problem.

When you have an ignition that acts up when it rains or is foggy, it is a good bet that you have a bad insulator somewhere (cracked cap, bad plug wires, etc.).