Can't really understand why the capacitor would respond to the well
aimed boot. It would seem much more likely to me that a terminal
somewhere is not quite tight - the boot would resolve that as would the
operation of changing the capacitor as the offending terminal would
have been messed about with. Plus this is quite likely to be
temperature related. If you've got access I would go round all the
connections and check them for tightness.

The other thing I would try would be to get some lubrication into the
motor bearings; they could be sticky as you suggest and the kick frees
it. It is possible a new capacitor could overcome this by the very
fact that it is new and just that little bit more effective. If you're
up to changing the capacitor yourself look in Yellow Pages for a local
company who do motor rewinds, that's where I got one when I blew up one
when messing about with the start timer on another motor.

Rob