It could be that moisture is getting into the electrical connectors near the rear of the transmission that I described in the following post. After experiencing a similar problem awhile back, I cleaned those connectors and waterproofed them as best I could and haven't had the problem since.
http://www.vehicross.info/forums/sho...3&postcount=13
If that's not it, there were a few other suggestions in the thread that might help too.