Is it taking alot of speed to hold knife edge ? You have 2-225's on the elevator ? All I can figure if you've checked all the obvious is that your airspeed is too high, but that doesn't help much..not like you can slow it down if it won't hold knife edge. Maybe a higher torque servo would be in order. Nothing like throwing money at a problem. One other option might be braces or flying wire on the tail...the whole stab might be oscillating and not just the elevator.