I've been vooming since November. I am on my third STB, all three of them had lockup problems, including the "New" one I've got on Saturday. I can tell you about lockups!
First of all, it seems there are at least two different types of lockups.
Type One, I believe is caused by bugs in the software (firmware). This type of lockup usually happens when you switch channels, or navigate in the PG. The more often you switch channels, the more often you get locked up. It also seems to happen more frequently when reception is poor, but I am not positive. Almost everyone experienced this type of lockup. If this happens to you, just press the power button for a few seconds or unplug the STB. And yes, you will have to wait a few minutes for the PG to load. Until the new firmware is released, not much else you can do. STB exchange won't help. At least it didn't help me.
The Second type of lockup seems to be caused by some hardware problems. According to Wilt, some of the older STBs had an internal defect causing this problem. The symptoms are similar to the first one, except that the problem rapidly escalates to the point where you get lockups every few minutes, and even without touching the remote. This is what's happened to my first box. It worked ok for 3 months, but then it just started to die. It would lockup on its own, either getting stuck on a single channel, or with a black screen. Occasionally it would even lockup during the power-cycle reboot! The only cure for this "lockup disease" is the STB replacement. If your STB locks up every hour without you touching the remote, call VOOM and request a new box. No need to wait - it only gets worse!