M600 97' going onto one cylinder

Hi,
I've had my problems after rejetting but i've got it running smooth now, i thought. After 2 happy weeks I went out in the rain and my monster worked just fine. The day after that when i fired it up worked as intended but after a couple of seconds on the idle dropped a cylinder. And it didn't help revving. I replaced the spark plug on the horizontal (the cylinder that didn't want to play) with a clean spark plug fired her up and went straight away without letting her idle. This worked fine and the idle was no problem when she got warm but after leaving the bike a couple of hours and then starting it again gave me the same problem. It idled without problem for a couple of seconds and then the horizontal cylinder felt like taking a break.
This have happened 3 times now in a row after i've been out in the rain/after it have rained, when it's a bit moist outside and wet on the ground. I start her up and if I don't go riding away directly the horizontal cylinder stops beating and i have to wipe/use a clean spark plug. There's no problem when it's warm it's just when the engine is cold and it doesn't seem like the problem arises when it's been dry outside or I've avoided driving when it's raining.
First I thought it was something with the carbs but the plugs look good as soon as the horizontal cylinder works... So my first guess would be an electrical problem?
Brainstorm!