If the screen is entirely black when the picture goes away (ie not a horizontal or vertical line), this is more than likely dry/cracked solder joints on the connector that joins the motherboard to the Power/Sweep board. Resolder all those pins (on the Power/Sweep board) and don't forget the one that is spaced one pin away from all the others at one end - usually the culprit.
The cable that plugs onto that connector may also need cleaning or just re-seating.
WARNING: There are live parts inside! Take appropriate measures and be careful!
This pdf might also be useful:
macintosh_plus_sm.pdf