I'm having trouble solving my transmission trouble. I hope someone can help.

1994 Nissan Pathfinder SE

Problem:
My transmission intermittently misses shifts. Seems to happen more frequently in the summer.

Problem Re-creation:
Only happens when the transmission light blinks upon startup.

Solution:
Took the truck to get the error codes out of the transmission control computer, and there were no error codes stored.
Of course, the light won't blink for the mechanic, so maybe the error codes get erased on startup after a passed test.
I'm hoping it's just a sensor, but which sensor. Hope it's not a wiring issue - could be a nightmare.

Thanks,