Thanks, I've now tested OpenATV 6, and it was the only fresh image that allowed me to restore the system settings from an OpenMB backup, so lamedb is the same, and no scan done, and it behaves the same.
What is weird is that some channels on one of the problematic muxes works fine, while others, on the same mux, pixelises like crazy, unwatchable.
All unencrypted channels work, while the ones that doesn't work properly are encrypted and is using a CI+ CAM, with an official card. Not all encrypted channels have a problem, but I think that all encrypted HD channels has problems.
It felt a bit like something running out of steam, but top said the box was 96% idle when it was pixelizing.
So now back to the old OpenMB again...
If anyone was any ideas how to debug this, I would be happy to try things out.
As all recent images seem to have this issue, it looks like it is some common issue.