Had the same problem last summer. After much testing/tracing I narrowed the problem to a failed control switch. Ironically two of my three fans dirt napped within days of each other. Called Fantastic and detailed my test results to them, and they immediately sent two new style switches. Along with the instructions and wiring diagram (sent via file attachment to an e-mail).
The replacement work will require dropping the control face plate down but be careful to not yank the wiring too hard. There will be adequate slack in the wires to let the face plate dangle while you remove the old switch and install the new one. Be careful to follow the color of the wires and install the new switch exactly as shown on the instructions.
By the way, the switch that I'm referring to is the "0-1-2-3" rotating knob switch.
After the replacements, my fans seem to run faster on the "1" setting. The third fan that did not fail (yet) has a slower fan speed on "1" setting.
I believe the switch failure boiled down to that little resistor that is positioned under the tiny screened hole in the plastic that apparently was intended to cool it. The new switch design does not have an external resistor, so the "cooling hole" won't be needed. I did not tape it over, because when the lid is closed, rain won't get in.
While you are in the fan restoration mood, you may want to do a couple of other preventative maintenance things too. Put a tiny drop of WD-40 down the lid cut-off plunger. Push it up and down a few times to work the oil into it. That helps break up the corrosion. Also wipe off the rain sensor plate with alcohol. If you spend a lot of time in dusty locations, that sensor plate will get coated with enough dust to make "mud" which will hold moisture and of course the fan electronics will assume it is still raining.