Since this is working on two STBs, it occurs to me that some how the STBs are crashing (they are special-purpose computers afterall). There may be something in your home that is causing a spike on the power line that the STBs can't handle (even thought a good design would overcome such things).
Is the STB plugged into a filtering power strip for power? That might help. Is the room temperature where the box is high (it's summer)?
Summer temperatures may an electronic device to overheat and cause it to crash, or aggravate a devices susceptibility to power (current) spikes, or can cause other devices to cause larger current spikes (like a freezer or refrigerator for example).
Hope this message is late and that your problem is already solved. If it's not power and it's not heat, then the only other things to know is if the problem is in the box or the room. Swap the suspect STB with one of the working STBs from another room. See if the problem follows the box or stays in the room.