Actually, the ignition switch is a posible. Just for funsies, drive over a
bump with the heater fan running. If the fan stumbles too, then it's the
switch.....most likely it is loose....If you get no results from this,
disconnect your ground from battery, then disconnect,clean and re-connect
every electrical connector, spade, multi-pin or otherwise you can find. Do
this indiscriminately as you should do this periodically as a preventative
maintenance item anyway.
Recently I had the same symptoms and it turned out to be the hall effect.
This also turned out to be the last thing I tried replacing, after hours
of hit and miss.
Good luck! Miguel in Durango, Co.