If a slap temporarily fixes it, then it's more likely that you've just got bad joints. Your friend should be able to reflow them. All you need to do is remove the analog board (mounted on the side of the Macintosh) and reflow the video/power connector at J4. It's at the center of the analog board and leads to the motherboard. However, it is recommended that you replace your capacitors to keep your logic board from being corroded by the capacitor fluids, which have a tendency to leak.

If/when replacing C1, Make sure it is rated for high voltage, otherwise you'll have lovely capacitor guts splattered all over your Mac.



http://www.jagshouse.com/analog.html