91 Legacy L Fwd - transmission or electrical?

I am working on a 91 Legacy 2.2 non turbo, FWD - the transmission fluid is cherry red, pan drop no metal or clutch material ( was changed last year ) and it recently started to "slip" when warm. Shut the engine off and turn back on it seems to be okay for about 30 seconds and does it again. Speed sensor 1 and 2 codes - replaced speed sensor 1 which controls shifts and line pressure, didnt do anything for the initial problem. How do you check the TCU input to rule the control module out? If not the TCU, then could a piston be cracked inside the trans, and if so wouldn't the clutches burn as a result of low pressures?

just rebuilt my 90 legacy fwd auto trans. last fall. It was doing the same thing after it warmed up intermittently. I still get a code #2 speed sensor. Seems to be working ok. I heard the #2 speed sensor is in the instrument panel somewhere,dont know where. The high cutches were a bit burnt. but still running ok after i rebuilt it. about $300 dollars or more and a lot of time and patience.