All transmission pumps are capable of producing more fluid than the transmission needs to apply the clutches and bands, lubricate the moving parts, fill the torque converter, and run some of the fluid through the cooler in the radiator. The excess fluid unseats a spring-loaded pressure relief valve and recycles itself back to the inlet side of the pump. If the relief valve happens to stick in the open position, all the pressure is lost and none of the clutches get applied- - - - -no drive, forward or reverse. All it takes is a microscopic rough spot or a piece of debris trapped somewhere in the relief valve assembly to make for an intermittent pressure loss situation. Virtually all transmissions have a pipe plug on the outside of the case where a pressure gauge can be installed with a long enough hose to monitor main line pressure on a test drive. I'd strongly reccomend that type of diagnosis. I've been working on automatic transmissions since the late 1960's and I've done several hundred test drives with one or more pressure gauges monitoring various functions. Good luck!
Jerry
