The Gaunt Man

It can happen when I'm just standing still with the toggles running, although it can also happen when I'm fighting mobs (I first noticed it fighting mobs, but I checked it standing still to make sure it wasn't one of my fighting powers causing a major drain).

Nope, sure don't.

I have 8 toggles that I turn on regularly – Fire Shield, Plasma Shield, Blazing Aura, Hover, Combat Jumping, Tough, Weave, and sometimes Fly. It was fine for a while, then suddenly my Endurance went haywire. It will be okay for a while, then it will dip large chunks and keep slipping down more than it recovers, and occasionally it will drop by 50% suddenly. If I don't turn one of them on, it doesn't happen, but as soon as I turn all of them on, it doesn't matter if I turn one off, so it's not simply a matter of cumulative endurance drain.