First place I would look...

There is a thermal switch on the intake manifold to turn off your overdrive if the engine overheats. Over time the connection on it loosens and it becomes intermitant.

Its a single wire sensor near the thermostat.

Kevin C

87 Turbo Intercooled Raider, roller cam, torsen rear diff, LSD front diff, lockup auto with modified converter, V6 brakes, low transfer case gears...