When it is hot out my wipers stop working after 20 min. When it is cold I have no problem.
Okay. Try this one. When the wipers stop working, try running a temporary ground from the motor to the frame (try a jumper cable). Frequently these things lose their ground. If this works, simply take a wire and make a new ground. It's also possible that the motor is getting old and overheating, in which case you need a new one.
