Elevator Stall
Thanks for all the replies. Mine isn't snapping; it is doing sort of a half-corkscrew ONLY at the top of a split-S, and then not all the time; weird. Elevator halves are synched, and throws are well within specs. I could be feeding in some aileron without realizing it.
I've since found out that while this phenomena (elevator stall) does indeed exist, it is rare among full-scale and practically un-heard of with models. What I was thinking of as elevator stall is actually the wing stalling due to excessive elevator throw causing wing AOA to go past the point where it stops flying.
Looks like I was half-right... :-(
Ollie, this was happening at low speed (the split-S was executed at the top of a 45 degree climb, to do a sort of FAI turnaround), so I'm pretty sure the servos weren't stalled. I hope not; I've got a 4721 (120 oz/in) on the elevator.
Steve