It looks like it is hardware related and probably cannot be solved. Unfortunate Sam hasn’t responded in the last 11 days on your question. More info can be found here:
Some bad news at the end, imx6 in general related. So it’s obviously not only the vero that is affected.
mk01 from xbian wanted to implement a reference clock on imx6 and found out some really bad things.
It seems the chip is neither able to display 50 fps / 60 fps over a longer time, but does internally just throw the pics away as it gets overloaded.
So whenever a vblank should come, normal hw would swap buffers to display the new picture, but imx6 is not fast enough to do so (at the above rates). So the HW itself just skips that frame entirely. It even does not report this vblank … when another next frame is ready and hw feels like rendering it it swaps and reports this vblank.
I think you figured what would happen if you would sync on the vblank …
mk01 has found the place where the skips are force introduced and now could echo the vblank signal never the less … but yeah … getting 60 vsync signals, while the hw only renders ~ 40 … just sucks. This can even explain slow menus and currently I don’t see any fix for this, probably running the display in no more than 30 hz?
And another posting by fritsch (cleaned without the ranting):
Is it forbidden to say, that the vero has hardware incapabilities that are never going to be fixed? That this GC880 dual core is even too slow to play DVD images even with all of his two cores used? That it is not able to play LiveTV content in double rate that even ATV1 does - standard television? That the wireless is so fucked up, that it’s unusable for streaming? Even the spdif cable does not fit into the plug … That it’s not possible to render even the normal skin with > 48 fps over time. Disable dirty region and watch the sysinfo page …
Sorry, if one is not allowed to say the truth: That you are selling here a box, that will never do what it’s spec promis ? A former poster has linked at least 3 threads, that are open, not fixed since several months - and the worse here: that stuff is not fixable, cause the hw is not capable of doing it.