When the console is hooked up directly to the TV this is just a brief flicker, through the OSSC it takes a few seconds. Is there anything I can tweak to fix this or is this simply an unlucky case of my TV being intrinsically slow with this over HDMI vs SCART/Component?
That 2 second delay is just inherent to HDMI/DVI whenever it has to resync. My Sony also has near instant switching between 240p/480i on SCART. That is probably because it processes everything like 480i so it knows what kind of signal to expect.
While the normal and Low-Latency version of the Game Cube Game Boy Interface software work fine, the Ultra Low Latency version seems to have sync problems. For instance, when playing Metroid Zero Mission it’s very visible in the background where the individual pixel rows just seem to twitch around when moving left & right. Is this a display incompatibility with the odd refresh rate or a bug/limitation of the OSSC?
This is not caused by the odd refresh rate but is actually a bug in the ULL version. I even experienced the tearing on a CRT with GBI ULL. It usually happens after a few minutes.