a6 ellectric window problem

i am having problems with my passenger window sometimes it works ok and then
sometimes it only comes part way up and shoots back down. to get the window
fully up i have to do it gradually a couple of inches at a time but like i
say sometimes its ok.
i have took it back to the dealer twice now i checked it when i collected
the car seemed to be ok but an hour later it was playing up again the garage
said they have stripped it right down and put it back together i was
wondering if anyone as any ideas what this might be before i ring the
garage bak up on monday and get fobbed off
cheers
stef