Video left/right jumping glitch with RGB

[system11]

Still having lots of 'fun' with now three grey +2s.

Unit 1: perfect aside from noisy audio linked to screen brightness
Unit 2: same audio as above, occasionally jumps approximately a whole character block to the left then right again
Unit 3: quiet audio, does the same left/right picture jump but more frequently.

The buzzy audio appeared on units 1 & 2 after fitting TR4,5,7 the right way around - new replacements. They have a better picture than unit 3 though.

The picture jumps are momentary like they happen for a single frame, it's always left/right, the whole screen including the border shifts. Sometimes it sits there for 5 minutes without flinching, sometimes it's randomly up to half a second intervals. I've done the 'finger test' I'd use on arcade PCBs trying to hunt for any component that makes the effect worse when it happens - can't find a thing. Unit 3 has one bad RAM chip but unit 2 sat in soak for 30 minutes passing every time so probably unrelated.

Has anyone seen this glitch before?

Edit: more info found - if I feed the signal to my RGB capture card via a powered splitter, the image on the TV still jumps as before, but the display on the capture is rock solid, so something about the signal going to the TV is considered borderline and varies across the 3 motherboards.

[system11]

Found someone on Youtube with the same problem and cable - mine does it much, much less than his.

https://www.youtube.com/watch?v=-oxdM8sENEI

[system11]

Made some progress - the TV is getting confused and trying to switch the colour system rapidly. Question is how does it work it out, I notice the CSYNC level is highest on the machine which does it the most. Not a huge difference, about 0.3v. If I set the colour system manually (to anything!) then the problem stops, screen position is different depending on the one picked. Put it back to auto and the screen starts jumping around again.

[1024MAK]

The video signal is not fully compliant to the broadcast standard. For example, the signal is not interlaced, the same frame is sent (with different content) every time instead of an “even” and then an “odd” frame sequence (as per the standard). There are other differences as well...

Hence some TVs have difficulty auto detecting the signal and get confused.

So it is best (if present) to manually select “PAL” and “50Hz” modes. Also if using a SCART lead, where possible manually select “RGB” mode.

It is possible to reduce the video and sync levels, but it is unlikely the levels are the cause of the problem.

Mark

[Guesser]

Have the +2s had the jumpers changed to set the RGB port to SCART, with the blanking voltage resistor changed to the correct value, and a correctly wired SCART cable used...?