pseudoswede I can't see any correlation between the TV OS version and the way your Ultra 4670 responds. There's nothing the TV can do through HDMI to force your Ultra to reboot. More than likely there's something else involved. If all the previous steps haven't resolved anything, a factory reset will clear any possible corrupted code in the installed apps and start you over with a clean slate. You have to set all your channels back up (they will reinstall automatically) but it's a one-time effort.
I must also point out that the 4670 is approaching five years old now. For an electronic device, that's no spring chicken. It's possible that something internally is failing, and there's really no cost effective way to fix something. I too have a 4670 which works just fine, but it also doesn't get a lot of use.
Especially since your player shows no sign of overheating, there's likely not much else you can try, other than the factory reset. That's always the last option to take, since it does take a little time to set your apps back up again. But sometimes it's the only thing that will clear random issues like this.