There are guides inside the door that keep the window in place during locomotion. These guides can be made of felt, hard or soft plastic. These guides can sometimes be contaminated by sound-deadening or corrosion-protection sprayed into the door, or the lube the window mechanicals ride on.
I've found the best way to clean these are to take the inside door panel off and access these guides for cleaning. This can be quite a chore to someone not familiar with the intricacies of removing or reinstalling these panels.
There's one detail to cover when doing this. Along the top edge of the door, where the window enters and exits the door, is the obvious rubber gasket. The INSIDE EDGE of this gasket is usually felt lined and can/will pick up any of these contaminations as the window goes up and down. Be sure to clean this inner gasket edge thoroughly at the same time you're cleaning the guides! It's a thankless job as it can take a LOT of window cycles before you've got everything cleaned.
Be very careful putting everything back together as this step is where many novices accidentally recontaminate the window or mechanisms again.
Good luck.
