It's beginning to sound a lot like the power supply is failing on your SB Pro-HD. There are several threads in the Pro-HD forum describing symptoms very similar to yours and how to get a replacement power supply -- for free if your SB is still under warranty, or for cost if no longer under warranty.
I believe from what you have said, that you are also being affected by the latest terrible firmware update. For some reason, slingbox has decided to make the boxes default to a static ip, usually 192.168.x.254. This happens at the end of the setup process.
So, when the box is set to a static ip, it will not show up in your dhcp list.
I have yet to figure out a workaround with this issue and my box is totally unreliable since this happened.
I do have my SB set to the static IP address -- did this before the firmware upgrade. It has been quite reliable for me, but then, I not only programmed my router myself to set a static IP address of 192.168.x.254 for my SB, but I also programmed my router to forward port 5001 to that static IP address. Since doing all that, then rebooting both the router and the SB, plus insuring that there was plenty of cooling air moving around my SB, which was physically quite warm to the touch, it has worked fine.