Wolfgang,
Your switch sounds like it's on it's way out. Start by changing out the window switch with one from another window. If that works, then the switch is likely your problem. If the problem persists, the you may have to change the window terminal block. One of these can be had at a junk yard but be sure to remove the harness back to any clip or plug. If you try to cut and splice that many wires it can lead to problems.
Good luck
Frank