Unfortunately this isn’t the same problem as I have. In the thread the setting DTV 480p fixes the problem. I’ve done some testing and realiced, that DTV 480p causes the sync issues. It runs perfect with “Auto” and “Vesa”. But with this settings the picture is too narrow and it’s not centered. This is why I bought the OSSC, because besides the Framemeister it’s the only device, which outputs the right aspect ratio with the Dreamcast.
I tried it on my PC monitor and another TV, always the same. On the monitor I didn’t have the snow screen, it went right out of sync after 30 minutes. Everything is fine when I put “480p in sampler” to “auto”.