It sounds like the mechanism - both my rear ones had this problem, basically the little clip that moves up & down the cable when you press the button is plastic & breaks. When you press the button the cable (inside the door attached to the motor & the window lifter) starts to move & the plastic clip that is attached to the lifter channel catches is it but gets so far & slips, then catches, then slips etc. etc.
You'll probably find it you push or pull the glass it'll go first time!
The plastic but is attached to the lifter channel which I think to buy new comes as a complete mech, motor & all, read EXPENSIVE.
Best bet, & what I did, get down the scrappy & just get the bit you need! The bits the same on all MK3's front & back however the rest of the mech varies.
Take the door card off though & operate the window so you can confirm it is this that's wrong though, hope that helps!
