NMS-8250 video issues

try return to 220V
check the power supply +5V
check the solder of the scart
check the sync signal :verify Q314 2SC536, IC 302 74HC00, C301 220µF
check also the 5V on pin 16 (fast switching) of the scart and the Q315 2SC536
and the 12V on pin 8 (function switching) of the scart

No I've not checked anything at all yet, except for switching cables and screen. It worked fine before, and it hasn't moved, neither have the scart connections. Now I'm the last to state nothing can break when you don't touch it, but I don't think the actual connection is the issue here. However, I'll open it up and resolder the connectors anyway.

As per returning to 220V, I don't think that's a good thing to do. The high voltage transformer is right next to my driveway, and I think that's the reason I actually measure 236V - 238V on my wall outlets. I'll measure voltages inside though.
Pin 8 is OK I think as the screen switches to the input when I switch on the MSX, and it stays on that input until I switch off again. The sync signal to the videoboard is OK too I guess, as using composite works fine. I assume to build up the composite signal, it needs those signals as well. I think it's in the very last part before going in to the scart connector. But I'll check all signals you mentioned. Alas I don't have an osciloscope anymore, the poor old thing died on me. Hope to get some time tonight

PSU delivers 5.00V exact on the 5V line, 11.90 on the 12V line. That should be in spec. Scart connections all measure out OK but I will still resolder them. I noticed when I switched it on today, it took like 6 or 7 seconds before it started glitching. After that when I switch of and on again, it starts glitching immediately. So that triggers me even more in thinking I'm looking for a bad cap. I'm going to measure them all now.

[edit]
I've just resoldered the SCART connector, that doesn't change anything. I've had some more time to test things. It only happens when the RED signal is needed. Ie. color 15,4 and 15,0 15,2 and 15,3 work fine, stable image. Color 15,8 and 15,9 for example make it go berzerk. So I looked at the service manual (I seem to have a bad copy, but it's usable) C303 and C306 are fine, but I still replaced them. So I'm looking at Q308 now, maybe bad transistor?
As far as I'm concerned, it's not SYNC that is lost. The image is still stable, ie. not wobbling or going out of sync, but just losing information. Now I must say I found my schematic reading skills have drastly gone bad over the last 20 years or so. But when I look at the sync signal, if I read correctly, that would have nothing to do with individual RGB components. It comes from the CSYNC signal from the VDP. While composite sync works, and the image stays in sync in SCART too, I ASSUME that part to be still ok.

Does my thinking make sense? What could make the image go bad when a specific color kicks in?

Is that a B&O TV? Have you tried another TV?
It looks like it's switching between RGB and something else, could be trying to switch to S-Video.

A non connected Fast-Blanking pin (SCART pin 16) would leave the TV in composite mode, a fluctuating voltage instead would cause the RGB/Composite switching.
However the black screen + strange colours shown in the video doesn't look like composite.
It could be that the TV is set to default to S-Video instead of composite, that could explain the bizarre problem with the red (as the chroma signal component is transferred via the red connection), or the TV could be defective. Sorry OP, we need more info.