Boxster windows out of control!

[PKLD]

Hi all, a work colleague is struggling with his Boxster S. After 2 visits to auto electricians the side windows still have a mind of their own and are lowering at random.

So he can come out in the morning and they're down completely, while driving along they drop, but he can raise them as normal (no noise so not like when my window regulator failed) but then they'll drop again.

Weird how they sometimes stay in place for a few minutes or a few days and always go up when asked!

Any ideas before he takes it into the Porsche centre for a big bill? I think it's a 2003/4 model