I've seen weird windows behavior like that before on work computers with a short in the video cable. Windows, for some weird reason, when it loses the monitor reverts to a lower resolution, and once the monitor connection is restored the windows have been moved.
I don't think yours is the cable though, I'm leaning towards the switch. Whether the switch has a signal or not, it should not change inputs on it's own. I've yet to see a switch or receiver change inputs simply because a signal is not found. That could make programming your remotes tougher because of simple power up order.
Try these things and see what happens:
1. Hook the PC straight to the TV. Does the PC still drop signal?
2. Place the PC on input #1 of the switch? Does the PC still drop signal? If not, does whatever is connected to input #3 drop signal now?
3. Try a different HDMI cable. Does the PC still drop signal?
4. Is there anything that could be controlling the switch, sending it to input #1? (script on the computer, neighbors remote, etc).