it's either the regulator, or their is a clip along the side of the
window track (yours may or may not have one) that helps hold the
window from tilting.
If it works when wet you may be able to oil/grease the regulator/track
to fix it.
also, a clip or something is probably broken since the window does
tilt. It shouldn't do that at all. It should only go up and down.
Try spraying some silicone spray lube on the window tracks in the
door. (on both the rubber and the felty stuff)
If that doesnt work it's time to tear the panel off and poke around..
HTH
BHen