It could be the window switch going bad. You can test by switching it with the passenger window switch( assuming that one works ok).
If that doesn't correct the problem, I would remove the door panel and investigate. I have had a couple window problems. The metal regulator cracked at the lower mounting point and completely failed.
Be careful taking off the door panels especially if you have power seats. Let me know if you do I will give you some tips that will make the job go smooth.
1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T