The transmission began shifting continuously between 4th and OD and back, endlessly, under light acceleration. After a complete Transmission and Torque Converter replacement, not just rebuild, the problem persisted. Another mechanic discovered that it was neither the transmission or torque converter, it was voltage.

When the transmission went into OD, the engine speed would drop. Upon light acceleration, it would pop out of OD, but then go back in, only to pop immediately out.

What was happening is: when the engine revs would decrease, the voltage to the transmission controller would decrease, dropping voltage to the computer, which then went calling for more power, and therefore, more engine revs, and the computer told the transmission to drop back to 4th, to speed up the engine. This would as expected, increase voltage, and the computer would shift the truck back to OD, the revs would decrease again, and the voltage would decrease, and the cycle would repeat every half second, endlessly.

A new alternator resolved the problem. A very surprising remedy for a failing transmission.