It's not surprising to me...This is called jitter...Use any Line 6 modeler long enough and likely you will come upon it...The causes are many and it is inherent with what is going on...

Basically, the analog pots are diced up into digital values...say 0 to 100...each it a specific voltage...When you move the pot, that setting is sent to the modeler, when you recall a patch, the knob might be in a different spot than how you saved it...A little vibration, slight variance in input voltage and the voltage varies enough that the pot sends its position...

This is something as a longtime Line 6 combo amplifier user that I have had to deal with...Much more rare that it happens to a POD, but it has happened to me...More on that in a minute...Anyway, the vibration in the combo amps causes this more readily...What a lot of us have found is that during a performance, park all the knobs to zero or max...I have never had jitter happen to me and it is basically a habit for me now most of the time...

with the POD I tend to leave the knobs...but I have had my HD400 jitter on a stage that had a lot of vibration...Even saw a voltage drop from a kick drum hit severe enough to make the HD400 reset...That was an intentional test I did during a sound check...I always using an AVR on my power supply, so I don't have to worry about that too much...

If this gets to where it isn't usable, it might be that you have a dirty pot or a pot that needs replacing...Hard to say...This is one of those things that can happen just because it is in the nature of the design...keep an eye on it...One thing you can do is hook up to Edit and that can help narrow down the culprit...