1991 S light blinks and check engine light comes on

I have read about many problems related to this same issue, it is either the speed controller or the TCU. The issue is kinda the same of
both but when the S light starts blinking, the check engine light comes on too and the car will start moving slowly. You press the gas and engine starts to rev high and the cat starts to accelerate slowly and picks up gradual speed. I have an automatic transmission however, if you shift the car into 1st gear then 2and and 3rd and 4th manually, the car will drive. When the cars S light is not blinking, the car runs and drive just fine.
I think that the TCU is going or has gone out but just wanted to see what people here think.
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