1997 Ford F-250 Sporadically shifts out of gear

The truck will run miles with no issue then all the sudden it will shift out of gear, what seems like nuetral. I can let off the gas and it will shift back in to gear but with a very harsh shift. The OD light will blink for about 5 min then the problem goes away, somtimes for a long period other times within the next 10 min. The odd part is if I am towing my boat or I just leave the plug adapter in the socket the problem does not occur.
Example: I drive 40 miles to work and back. Probably 3 days out of 5 the problem wont occur. It does seem to happen the longer I drive it. Almost as if something has reached the hot point and is shorting or not working correctly.
Any Ideas? I have replaced the VSS and the MPLS. Thanks in advance.