This might be a long shot, but I had a cascade of incrisingly annoying things in the past week happening on my 8300HD+. Could be related, maybe, because my problems started with 619 RDI-HD refusing to appear (it'S on tune-on-start). I always had to switch canal, and come back.
And before I did found the cause, many channel just stayed black unless I downscaled to 720p (push the setting buttton, 4 times up, 1 times right, select).
The culprit was the HDMI cable. For some reason, it doesn't work anymore. Yesterday, It was causing freezes on channels, some other channels were black, andit even forced many reboots. I changed to the PyPbPr - A-L/R cables that came in the box, and all problems "magically" disappeared, getting 1080i back to normal, no more de-sync with audio or drop of frames, no more reboot, image is even better then it was.
I swear I kid you not, it was only the HDMI cable, signal strenght was never an issue. Is it the HDMI Port or the cable, can't say for sure, but it's one of theses two.