Hmmmmmm.... that's curious.

Have you resinstalling or updating the firmware on both your HD500 and your Variax? It might be a firmware issue on either device.

It might also be an issue with the knobs and/or switches on your Variax. If any of them are dirty and hence not making a clean solid connection with the Variax internals, it's possible that the electronics is detecting a phantom movement of a knob/switch. In that case, of course, the Variax will immediatley override the HD model setting and will begin to use the model indicated by the physical knob/switch positions. As you know, that happens immediately when you do physically move, say, the pickup switch selector. I don't know if it happens with the Tone and Volume knobs. I doubt it, but maybe worth testing. Definitely worth checking your Model Selector knob and your Pickup Selector switch. I would try just moving them slightly manually, without actually changing the model selection/position, to see if you can trigger a phantom model change.