I don't have HD PRO, so I can't help you directly, but here are couple links where you might find aswer:

http://line6.com/support/thread/83258

http://line6.com/support/thread/88492

http://line6.com/support/thread/77958

In one of these threads, rafgys is suggesting to another user this: "let me know if starting up without guitar plugged in, and waiting 30 sec- 1 minute to "warm up" the unit helps in your case at well (in my case, that seems to be the only way to avoid restarting the unit several times)"

I had the same problem on my Pod Hd Pro. The only way i lost this problem is:

Before you start the Pod Hd Pro, set the Master volume at minimum, and unplug the guitar cable (input)

Start the unit and wait around 20-30 seconds, the plug the guitar cable in. Hope this helps you.

I am using it like this for two weeks now and i had no problem. Before this, it was really fustrating, turning it on and off until it responded corect.

Hope this helps.