It is either a ground problem (probably) or the positive battery cable is
corroding internally and getting wet.  I have new ground straps for the
transmission - $6 each and new positive battery cables (seriously heavy duty)
for $70 each.  You could also check the ground on the engine (driver's side
head) and replace with a new one from Wal-Mart.  Hope this helps.

Thanks,
Ken Wilford
John 3:16
www.vanagain.com
Phone: (856)-765-1583
Fax: (856)-327-2242