This may not be the issue, but I had a problem where on another car the horn, cc, wipers would quit working then start working from time to time. Turned out to be a grounding issue where the cluster of ground wires connect to the neg terminal of the battery.
You might try replacing or possibly just clean the contacts on the High speed fan control relay in Auxiliary relay box 1. The box is in the engine compartment below the air filter box on the frame rail. It is a black plastic box about 6"x2"x2". The HSFC relay is the larger one in the center of five relays. Depress the two clips to open the cover.
Another possibility is the blower motor itself, but I'd go for the relay first.
