transmission not shifting into overdrive
 

I have a 1995 pontiac Bonneville with a 3.8 liter automatic and the other day it started nit shifting into overdrive for me. It seems to do this after the transmission warms up. Also once it starts doing this it seems if you stop for a few minutes and shut the car off that it will start working properly again. It makes no weird noises and shifts fine through all the other gears when this occurs. The fluid level is fine but I am not the origanal owner of the vehicle and am not sure if the filter and fluid have evcer been changed. Also it appears that the outside temp makes a differance, the cooler it is outside the less it happens but once it gets over 65 degrees outside it happens more often and is less likely to work properly. What could be the cause of this problem?