That sounds like the motor and gearing is under more load than it should be. First off, try lubricating the window guides with a bit of PTFE spray (e.g. GT85) or silicone spray (WD40 do a silicone spray now). Spray down the guides, and down the edges of the windows with them down so it runs inside the door, and run them up and down a few times (top tip, have the engine running, those window motors draw a hell of a lot of current). Wipe any excess off paintwork and plastic trim (the window rubber seals will benefit from a smear of these sprays).
If that doesnt significantly improve it, then its door card off and lube the mechanism (check for corrosion, and blocked drain holes in the bottom of the door whilst you're in there). You can even try to lube the bits of the motor mechanism you can get at - liberal spraying on any axles, hinge points etc.
If that doesnt work, then it'll be a new regulator coming soon; do it before it siezes with the window open! Thats [email protected] inconvenient.