I've been away from here for a while but I thought I'd throw in my 2 cents since I've been experiencing a similar problem for some time. I still have two 8300HD's but I only see the problem on the one in my living room. I have swapped the two boxes and the problem persisted in the living room despite the fact that the signal level is slightly better there. Like yourself, it always boots at exactly the top of the hour during prime time.
After studying this behavior for a while, I realized that it tends to happen when two recordings end at the same time. So, I began adjusting the end times of my recordings to ensure that two never end at the same time. Since I began doing this a few months ago, I haven't had a top of the hour reboot. Wherever possible, I'll add a minute to one recording but when I have to, I'll shorten a recording since it's better than losing 3 minutes during a reboot.
So why does it only happen on my living room box? It’s because I never record two at once at the other location. I suspect that there firmware bug that causes this to happen.