Actually since your issues are dampness related it would be best to see if you can resolve the weak spark issue in the same conditions when it occurs. Just be careful you don't get ZAPPED!
What is your battery voltage reading both running and with the ignition off?
Engine to chassis ground could be the weak spark. Check for a voltage drop from the positive battery terminal (part on the battery itself) to the connection where the big cable meets the starter. The check for a voltage drop from the engine itself to the negative battery terminal.
Voltage drops of over .25 volt indicate there is additional resistance in the circuit that is not supposed to be there, Just take the leads and go to the parts and touch them with the meter set on volts (not ohms, it could fry your meter or blow the fuse). Think of a voltage drop as a plugged plumbing pipe and you have low water pressure. Starting at the battery, as long as it reads about 12.5 volts (not running) and 13.8 to 14.2 running. Then go to the first place the main battery cables go where there is a connection. The voltage should remain the same. If the voltage drops you have probably isolated your problem especially if it is more than .25 volt. Unintentional added resistance in the circuit reduces to power available to the electrically driven components. Low battery voltage can really make things go crazy, all kinds of stuff not working properly. The battery may be the problem, or the pathways electricity need to do the job could be the problem. It's usually at the points where things are connected to other things, especially in areas where corrosion is a problem.
Keep me posted and I will see if I can walk you through it. Believe me I have been there and done that, including buying cars that were partially submerged and fixing them so they could go another 10 years, but that topic could take pages.
I have had people get mad at me for not tackling these types of jobs, because there is no way you can estimate the unknown, just plug away and isolate the area where the problem is. You start at the battery and work outwards, fuse block, grounds. Remember the engine to chassis ground which is usually just a wire that runs from the valve cover to the radiator support. If you don't see one the put one on. The volt meter will show you it needs one if you just touch the engine and a bolt on the support that is plated. Scratch the bolt head to make sure you have a good connection. If you see a voltage reading from the negative battery terminal to the engine itself, you know there is not a good ground, add the ground (even a simple wire with two alligator clips, or just the bare wire ends under two bolts and eliminate the voltage reading it might fix the weak spark.
I'm not going anywhere and there are others here who know just as much, so you have the advantage of our common experience. When I was 16 it took me week to figure out plug wires could be bad, the second time it took about 5 minutes. Stupid graphite core wires!
regards
Mech