It's actually plugged into the loose bracket on the back of the computer, which has a cable directly to the motherboard. So I believe that would be the same as plugging it into the hub.
I haven't tried to unplug and plug in again because the computer is up in the attic, and it takes a ladder to get up there! I set it to playback a few hours ago, and now it has frozen totally. I can at least exclude heat as the problem, the motherboard temperature was around 20 degrees, and the CPU temperature was around 25 degrees (gotta be some kind of record for an air cooled AMD XP2600!). But the PVR USB isn't freezing because it's cold either. I have put it inside the PC case, where the CD drives normally are because the attic has mouse from time to time! So the loose bracket is actually laying inside the case as well, strapped to a cable so it can't short circuit anything.
Edit: I'm now gonna install the WinTV software to see if that freezes as well. It's useless anyhow, because you have no aspect ratios that will fit my 1440x900 LCD monitor, but at least it will help diagnosing the problem.