. Need to get out a diagram and study it when you have a stumper like this.
If you can have the car long enough to verify that this is happening, then look at the wiring diagram and decide what you are going to test when it's in the failed state.
In the case with the bad ignition switch, it acted very much like this. I could test the fuse in the fusebox for the blower motor regulator, and sure enough, when it was in a failed state, there was no power at the fuse. The power came directly from the ignition switch, which was intermittently providing no power to this fuse.