Thank you for using just answer.
There could be two problems.The first one is the drivers seat motor.The motor could be freezing up and causing it not to work.Second is the switch itself.The contacts in the switch could be failing.The best way to see which one it is,is to have a test light or volt meter handy.When it messes up check for power at the motor and switch.If it doesnt have power coming out the switch then you have a switch problem.If it has power coming out and going to the motor then you most likely have a motor issue.
I am going to send you a wiring diagram for you
CLICK HERE for download
Thank you
Carl
Just Answer Expert
Ford Technician
6 years ford experience, Ford Master Diesel Tech, Ford Certified Tech