I hope I'm not jacking this thread.  Like the poster just before me, I have a guitar Pod XT Live (not the bass model) with the very same symptoms.  While playing, the unit will spontaneously leap huge increments in volume, either upwards or downwards.  I'll readjust my amp gain to compensate and then eventually, wham... off it goes to a radically different new volume setting.  I'm not 100% on this, but I think I'm seeing this phenomenon more often when the unit is cold then after it has been powered on for some time.

Took it to an authorized repair shop in Wheaton, MD and after $100 and several round trips, it now seems to be a little worse.

No active electronics, just a passive Tom Anderson hollow drop-top directly into the unit.  Cables are fine.

One thing I've noticed just by diddling with the thing, sometimes, immediately after one of these spontaneous gain changes, if I either (1) whack the volume pedal with my foot or (2) wiggle and reposition the "Drive" knob, the unit gain will restore to what it was just prior to the madness, but sometimes not.  Last night I updated all of the unit software, reflashed the unit and calibrated the volume pedal.  If I knew of any other possible remedies, I'd try them as well.

Is anyone aware of a calibration or reset procedure for all of the rotary encoders on the unit?

Because the unit is so flaky now (despite being well taken care of - no drops, etc.), I can't trust it in a live performance situation.  The changes in gain are so prominent that it's disruptive to any performance.  Also, like one of the other posters earlier in this thread, it's also becoming too much of a pain in the *** to practice or rehearse with.  It's a shame because I am really fond of some of my home brew user patches, but I'm now afraid to use the thing.

Any deep knowledge Pod XT Live medicine men out there?