Seein as i have to work with replacing door glasses most of the day, firstly lubricating the front and rear channels helps a helluva lot, and also popping the door card off and re greasing the runners and the runners poppers.

Also come across another problem every now and then with the secondary runner guide that holds the glass level (two 10mm bolts holding smaller runner to rear of door)

Re-grease this runner, and crack the nuts so the runner goes slack, move windo up and down once or twice, and adjust the runner accordingly to the best movement. common problem with the secondary runner moving for no reasonafter