This sounds like a Grounding issue. Check the Body Grounds, Remove them clean them and Seal with Dielectric grease. My Guess is when your going up a hill the wire is pulling off its mount just enough.. I have had lots of crazy stuff like this with bad grounds. You also have a Resistor on the Blower itself . they go bad over time. But that shoudl be bad all the time and when its bad the fan motor will turn very slow or not at all. . Check the Grounds.