I have a '79 with power windows. The driver's side window would not go down. Pressing the switch in either direction caused the voltmeter to dip, indicating current draw. I checked the connector at the motor, and had 12 volts, up & down. Suspecting the motor (it was the original), I pulled it and bench tested it. It worked, so I gave the gear a good cleaning, re-lubed it and re-installed. I went ahead and cleaned the connector contacts and applied dielectric grease while I was in there. Voila! Worked fine. Buttoned everything back up. Still worked. Great! Next day... window would not go down, same symptoms as before. I replaced the motor with a brand new one. It worked perfectly. Ran it up and down several times. Great! A couple of days later I drove the car with the window down and parked it in the driveway. After dinner, I went outside to put up the windows and, you guessed it... the driver's window would not budge. Again, the volt meter dipped when I Pressed the switch. Determined to not pull the door panel off again, I closed the door, started the car and after moving the switch a few times, the window went up. I cycled it up and down a few times and it worked fine. Any ideas as to why this window is so hit or miss? Bad switch? Sticky regulator? Ideas, please.

 

Guessing from your post the passenger side works when the drivers side fails. This eliminates their common connections like the circuit breaker and power..
1st swag is it sounds like the switch contacts are worn/corroded so that they're making intermittently. Swap the left & right window switches for a quick troubleshoot.
2nd swag is a bad door ground. When the window is acting up, make up a jumper or even try using jumper cables to run another ground from a good interior ground to a door hinge bolt to see if that effects it.
3rd swag. Try and trace out all drivers side power window circuits to make sure there's no damaged wiring.

One time while I was working on the door latch/handle on my 75 I lowered the power window with the small access panel off. This caused the window to stay jammed in the full down position and the window switch wouldn't raise it. I connected jumper cables to the battery, connected the neg to a bolt on the door and made a short jumper with a female spade lug so I could connect the Pos to the window motor connector. (one of the motor connectors raises and the other drops) When I applied battery voltage to the up contact on the motor it unjammed the window and raised it faster than I've ever seen it raise. So I moved the jumper and it lowered the window much faster..
Moral to this story, theres quite a bit of voltage drop in these C3's old wiring and switch contacts which slows them down and point two is to never operate the power windows with that small access panel off, LOL

Good luck

 

Can you replace the connector pigtail?

 

Before I would purchase any new parts I would remove the switch(es) and clean the contacts.
Youtube link:

 

Good suggestions, all. I have ordered a new switch, but will first try rebuilding the the original switch. I'm pretty sure the OEM switch is better quality than the new one.

 

I had a similar issue on my '70. Turned out the switches needed replacing.