Hi

Could be a number of things causing this.

Your XT Live Power Supply might be on the way out make sure it is the right one - i.e. 9v AC at 2000mA as it also has to provide power for the Variax.  The XT Live might benefit from a factory reset after you have saved all your presets to computer.  The VIDI cable and/or socket on the guitar might be playing up - check the socket on the guitar as the pins which make the RJ-45 connection do become flattened over time and this causes connection problems.  The guitar may be experiencing other issues with the selector switches that could be causing intermittent problems and lock-ups with the XT Live.

Remember these things are computers in the guise of everyday objects (stomp pedals and guitars).  Power fluctuations just as with a computer can give rise to all kinds of odd behaviour.  Maybe you have some kind of pwer conditioner available which would ensure that the XT Live gets a smooth input of power without possible dips and spikes caused by the fridge motor kicking in in the kitchen or beer pump at a live venue.

Intermittent issues are always the hardest to diagnose.  If you can, you need to try and get a reproducable situation where doing A + B + C produces problem X and with an intermittent problem that can be a nightmare.

Nick