1994 Toyota Corolla Trans Cut Out

I have a 1994 Toyota Corolla wagon. For the last 6 months the transmission has periodically cut out while I am driving for 10 to 20 seconds. I took it to the Toyota dealer and he test drove it for almost an hour with no problems. It happens infrequently. Then recently it happened again and the speedometer also cut out and went to zero. I was able to shift into a lower gear and drive that way for a minute or so. Then the speedometer cut back in and I was able to shift into Drive and all was well. I took it back to the dealer who did another test drive and then dropped the transmission to check for gunk or build up in the trans oil filter. None was found. He reinstalled the trans pan with a new gasket and topped off the fluid. He then did another test drive and all was still normal. I also had the vehicle speed sensor replaced but it is still happening. The mechanic is stymied and can’t think of anything that it can be. Any suggestions?

Exactly what does it do when it CUTS out,motor rev up,motor dies?
It is a auto trans correct?

Yes, auto trans. The transmission is suddenly not engaged. When you push down on the gas pedal the engine races like the trans is in neutral

Sounds like a bad seal on the forward clutch piston … requires a reliable transmission shop for accurate diagnosis.