It's skipping cause the plastic internals of the gear box on the motor are slowly breaking apart. Soon it won't go up or down at all. Open the window motor gear box and see all the plastic bits I'm talking about.
My fix was to remove the plastic "clutch *****" and fill it with JB weld, fixed it permanently and window moves smoother and faster.
Make sure you clean out all the plastic bits, them pack it really good with grease til it almost over fills.
The ***** are there to create a shock to help move the window from a stand still, but over time the ***** break down and slowly crumble till only bits are left to help move it.