I'll have a careful look at the wires in the door later tonight. It would explain why the switch would suddenly start/stop working for periods of time. Appreciate all the support.

So it's definitely not the switch. Tested against the working driver's side switch, same result. The switches all behave the same way--they'll work for about 20 seconds and then stop. I verified that all the connections were solid and the position of the door makes no difference. The sparking was an unrelated issue that must have been caused by wires touching; wrapped any exposed metal in electrical tape and no more sparks from any of the stitches.

The wiring in the door looks fine and the passenger side switch works reliably. If the two passenger window switches are associated, is it possible the problem is a loose connection on the passenger side?

Make sure all you connections are 100% accurate wire to switch ...

Check if the window lower support-rail is attached to the window.. if the motor is working and the window is not moving up.

Let us know what you find.

Thanks Kraftwerk. I'll double check, but I'm pretty sure they're accurate. The support rail seems to be ok, since the window operates normally with the passenger side switch. the only time the passenger side switch didn't work was when the driver's side passenger switch was completely detached, so I assume power runs through the driver's passenger window switch.

There have been times when the switch would die for a little while, but it would always come back and work for extended periods. This time the working time is down to almost zero. Every time I connect a new switch, it will operate for a few second and then stop as if the motor isn't getting enough power from the driver's side. The passenger side switch does the job with no problem. Maybe it is in fact a weakening motor, as a couple people have suggested.

Weakening motor combined with the spring not resting on its rivet ( "rivet fell-off" is super common! )
Awhile back, I got a new switch which looked really cheaply made, it had defects... not all switches are equal ~ !

I thought that too, but a known working switch does the same thing. I really hope I'm missing a simple fix. Replacing the motor would be a drag.

In case anyone is interested, the problem in fact turned out to be a defect in the passenger side window switch causing it to stick and eventually fail.

Replaced; all three switches working better than ever since.