As these windows usually see little use the grease lubricating various parts of the mechanism dries out and hardens. Sounds like it is time for a full clean and re-lube of the whole lot. I try to exercise mine every now and then. When marginal they are sensitive to voltage available, so idling won't help. Try to idle at 1500 or even 2000 rpm and see if it is just enough.

My general experience is that dried out grease in all parts of the system gives the symptoms you describe. WD40 has never solved the problem for me: Dismantling, cleaning and re-greasing has been the only way. Even the steel rollers, wires and tracks are stiff with dried out grease and collectively they are too much. Dried out grease in the gear of the window motors are a major snag too.