It sounds like heat is causing metal to expand enough to make contact. The first thing i'd check is the relay, which is located behind the glovebox and to the left. If any internal connections have failed, resolder them. If the relay is acceptable, check the connection on the right side of the transmission (passenger side) where the OD solenoid plugs into the wiring from the shifter. It is located above the pan by approximately a foot. You may have to unplug it and clean the connections. Good luck!