I've had this exact problem on my 406 (climate control model).
Thinking that the cause was worn motor brushes, I ordered two new ones brushed from ebay and soldered them in. All was fine for a couple of weeks then the fan intermittently stopped working again. Hitting the fan sometimes got it working again, but a couple of days later it packed up completely. This time I inspected the innards of the motor much more closely and I found that the copper 'commutator' (upon which the brushes are sprung against) was badly worn down so much that some of the commutator's copper segments (there are about 12 of these?) were worn right through and were therefore electrically 'open circuit'.
The intermittency was therefore due to the fan motor sometimes stopping (when you turned the car off) such that one of the bad segments was aligned with one of the two brushes, and when the car was next started, the motor wouldn't start because of the open circuit. However, if the motor was then hit, the force sometimes jiggled the motor so that the brushes just about made contact and the motor started. Once the motor was in motion it did not matter about the few open circuit segments because the motor had enough momentum to take the brushes past and onto a good segment.
Worn commutators are not repairable and the only solution was to fit a NEW motor, and this fixed the fault for good! I would advise against using a second-hand heater motor because they are likely to also be highly worn, unless it was known to be from a much lower mileage, or younger vehicle. Be sure to get a motor that is the right type - One type is designed to operate via an external resistor (to alter fan speed) while another type has an integral electronic controller to control fan speed. The latter type are fitted to 406's that have 'climate control'.
The heater motor that I used was brand new but had a different (thus newer) part number to the original, and looked slightly different, but was compatible.
These motor assemblies are easy to change. Remove the carpet that covers the motor under the glovebox, remove three screws to allow the motor to drop down, and then disconnect the connectors. One of these is a connector on a single wire - don't forget to reconnect it as it is easy to forget. I can't remember whether there were one or two further connectors, but one had four wires and was difficult to remove - I had to gently prise it out with a screwdriver!
NEW motors could be £200+ but I was lucky enough to know the right people and get it at cost price, around £60.
Justin Thyme
June 2010
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