97 town car O/D light comming on by itself

I have a 97 town car with apx 125k miles on it, I felt it down shift and the O/D off lite came on. the lite goes off, blinks,stays on when you start the car ,it does all these things...i thaught it was the transmission so took it to transmission shop...he put a new computer on the car(600 bucks) picked it up drove it for about a week and it started all over again.............took it back got a call it was ready picked it up after hours drove apx 4 miles and all over again...back to shop Can anyone help with this problem