Random Gain sudden changes

[ignotus]

If only the snare shows this behaviour then it stands to reason that's your problem right there: the snare itself. I'd try changing the piezo sensor. They can develop cracks/detachment of the ceramic part that can be impossible to diagnose visually, and don't show errors with a multimeter, because while testing with a multimeter it's not getting bashed around. These kinds of faults in a piezo can mean it continues to "work", but with erratic performance.

[dmitri]

I can only second what ignotus said about the snare.
Do other pads/cymbals behave the same?