the lack of overdrive

I have a '91 Lexus es250. The car has been good to me for the most part in the 5 months i've had it, but I was driving home from school one day and the Overdrive decided to turn off by itself. I turned it back on later on and the light started to blink and turn itself off again. So now i keep the OD off and drive slow. Does anyone know what could possibly be a reason for it to turn off? My mechanic can't tell me anything specific...maybe something to go and look at? Thanks.