I'm not sure if the Boxster has these, but in the 928 there are felt guides that the window slides between. If one comes off or wears out, it can scratch the window and the window feels a little loose.
For the up/down problem, try resetting your windows. Do a search through the forums here and on
www.renntech.org for "reset window" or either word to find it. It involves unplugging the battery for a few minutes, then plugging it back in and doing sequence with the window switches. That sets the upper limit of the window so it stops at that point. If it doesn't stop and feels too pressure because the limit is not set, it may go back down because it thinks something (like your arm) is stuck in the window.
Hope it helps.
