Okay, resoldered the J4 connections.

The rebooting problem remains, but i got something on the screen. A vertical line moving up.

Readings on the lines are crazy. They start out fine at 5.02 V and 12.02 V, but after about 10 seconds they start going up really fast. Untill the reboot. I've measured like 15V on the +12V line.

@nvdeynde: no, i haven't got a spare board...

*edit: here's a movie of the +12V line and the screen:

http://tinyurl.com/ovthkwf