over drive light blinking on 1998 Ford Mustang

I drive the car once it goes through one shift sequence the over drive light starts blinking and does not stop until i turn off the car when it is shifting (automatic) it shifts very hard I have checked the transmission fluid level and it seems perfect.

Asked by for the 1998 Ford Mustang
it sounds like something has gone wrong and your transmission has gone into default mode
is it shifting hard? and at a slightly higher rpm?