I followed the thread that Bibilit quoted and it worked for me. I had the jitters or blank screen on boot up... I had to hit the poor Mac a few times (Someone may report me for Mac abuse here!). Eventually it would be stable after 8 minutes.

I mentioned in that thread I was lazy just reheating the old solder joints which actually made the jittering worse - slamming the side of the Mac Plus no longer worked because of this. Anyway, some fresh solder and soldering joints that would have made my Science teacher proud, my Mac Plus has been 100% fine since! :beige: