if i were to compare this to something else "wrong" with the electrical motors in the Qx4 is compare it to the Windows motors; the windows tend to become sluggish when they roll about half-way down, to the point where maybe they stop moving altogether.
Usually members clean and then spray some silicon lube down the window tracks to make it run smoother.
It might be a similar case where the seat tracks get "gunked" up with the pre-existing grease and surrounding dust.
I might be wrong so I will take a look at my own seat-tracks to verify if this is a possibility.
The relay-click heard when trying to move the seat forward/backward (to me!) implies that electrical power is being delivered to the seat motor (assuming the relay contacts are in good condition), but the "gunk" is so thick that it prevents the motor from successfully driving the seat fwd/bkwd. So maybe all that's needed is that the tracks need to be cleaned and re-lubed or greased.
Questions contradicting my hypothesis above:
1. well, why does tapping or knocking on the motor case make it suddenly work again?
Well, maybe the motor is "tired" out. It could be that the motor isn't constantly producing enough driving power to move the seat. GIving it a few taps might give it that extra "umph" to move the seat, and once the motor is in motion, it keeps going.
2. well, how about pulling or pushing on the seat to get it moving?
considering the "gunk" idea and the "not enough power generated" idea, if you help the motor get past restrictive gunk, maybe it makes it a bit easier for the motor to move and drive the seat. It might be temporary until it get stuck in the gunk again.
3. what if there is no "gunk" in the tracks? what then?
well, maybe now it's the electrical delivery of power to the motor. Electrical motors, and I'm no motor expert - i'm just speculating, are driven with magnetic fields. Dust and whatever can build up around coils and wires that help produce electrical driving forces to move the seat. If the motors are dusty, try blowing some compressed air in it, like those you find in a can for computers.
Or if not dust, then the Power circuit boards that distribute and relay power to the motor. Usually, it's my believe that electrical motors don't go bad, especially ones that aren't in constant use. We probabably move our seat back and forth maybe once or twice a week? And on top of that, they're short motions. I doubt most drivers sit on their seat and move it back and forth just for the pure experience