I started having the misfiring, running on one cylinder when damp/wet a few months back. I found the fix on this site, can't find it again to say thank you!!
So the definative fix :-
Get a mountain bike inner tube cut two six inch lengths from it. Put the coil inside the inner tube, it is a tight fit.
Use self amalgamating tape to seal the ends.
Since doing this I have ridden through some really foul weather up and down the M1 and the TDM never misses a beat.
I was worried about over heating, but as it has been at least three months, in all temperatures so I guess this is not an issue.
I hope this helps someone else.
The blame is really with Yamaha - crap coils.
Edited by OldGreyWizard, 16 October 2013 - 08:26 pm.