HEre's the thing: it is unlikely that the same malady would develop in both switches simultaneously. If there is a single lead between the switches on one of the poles then I would want someone to check that lead for undue impedance. If it is not a common lead between the switches (whcih is possible but not likely from the sound of the problem), then the governing issue is the fan itself and themost liekly candidate is indeed the circuit board. One component on a small circuit board can screw the whole logic of the thing, although I can't quite figure the half-switch position thing.

A