The car is a '79 with power windows. 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. 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. 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? Maybe it's the regulator and not an electrical problem? Looking for input.

When I installed my factory power windows about 40 years ago, I seem to remember there being a relay involved. I don't remember exactly where it was located but I suspect it was under the console with the switches.

I'll try to look in the factory service manual when I get back. 

1979 would not have a relay. The power windows run from a circuit breaker in the fusebox. Could be a faulty circuit breaker, or wiring issue. 

If it doesn't have a relay, that would mean all the motor current would pass through the switch. In that case I would definitely check the switch or replace it. 

Indeed, '79's do not have power window relays. The entire load is carried through the switches. I'm reasonably sure the breaker/power source is not the culprit, as both left and right switches are fed from the same breaker and the passenger-side window works normally. I got a current draw when I pressed the switch in either direction, which led me to believe the switch was working. The only load on the circuit is the motor, and since the voltmeter is dipping when I press the switch, I figured the switch must be sending juice to the motor. Ditto, why I ruled out the wiring to the door. I suppose the switch could be worn to the point where its' internal resistance is above normal? At the moment, it works. It's probably waiting for a downpour to decide it doesn't want to go up, lol.  

Swap the connectors on the switches. If the ds window works but the ps stop working, then you know the switch has failed. Switches are in the console, correct? 

Yep, switches in the console. Good suggestion! 

Them buggers are noted for that. Don't get new ones from R.O.C. auto. Find O.E.M. ones. They'll last till your done with the car. 

If it turns out the switch is bad, I suspect it's due to contact resistance that's built up over the years.  The reason you got a current draw was because even with high resistance, there could be a minor amount of current draw but not enough to run the motor. 

Thanks, everyone. Good suggestions. I have a new switch on the way, but will first try to disassemble the old one and clean the contacts. Most likely the OEM switch is of better quality than the new reproduction.