96 jimmy blower motor

the blower motor in my truck works on high every once in awhile, all the other 3 lower speeds work all the time, but every once in a while you can kick it up to high and it works. i have replaced the blower motor several times, just this july the last time. can anybody tell me why it wont work on high all the time? thanks

Blower motor switch or relay.
Get a wiring diagam and a test light and see which one is not turning on.