PPC640 External Monitor Sync Problem

[Rodders]

I've recently acquired a PPC640 and I have plans to upgrade the screen and add some form of hard disk storage.
However I've run into a problem.
The built in screen works OK (as OK as it gets) albeit with a line of text near the bottom missing.
However, when I connect to an external monitor, either using an RGBtoHDMI or GBSC (with sync combiner) I get a strange flashing effect.
The screen displays fine for about 1/2 second and then it either goes blank or goes scrambled for 1/2 a second.
I've have attached a video to show this.

I was wondering if anyone had seen this effect before and could give me clue about how to fix it.
I've scoped the Hsync and Vsync lines and they seem OK if a little noisy.
Could it be a power problem due to failing caps?