I recently sold one of the best looking STEs in my collection. Very modern motherboard (square DMA), updated with new TOS and recapped PSU. Tested and working nicely with an UBE video adapter and the flatscreens I have.
But for the buyer it's not working as it should. When cold the image is fine, but after 10-15 minutes it starts to wave oh so slightly from side to side in an oscillating motion (see three videos linked further down).
The buyer has tested with two flatscreen TVs and one CRT monitor, all using an RGB SCART from Retro Computer Shack (https://www.ebay.co.uk/itm/250978089310
). The monitors work fine with other retro computers, and the cable gives a rock solid picture with another STE the buyer already had. The issue looks exactly the same on all of them.
I'm at a loss really to explain what could cause this. I would tend to guess at some HSYNC instability, but don't really know how to pinpoint it better. I've offered to take the computer back but the buyer would like to fix it if possible (it is, really, in excellent condition otherwise ... )