Hmmm. You have a good one there. I would start by plugging in another power source (receiver pack or even 4 AA cells) to eliminate the possibility that there's some short in the receiver pack that only reveals itself when the power requirements exceed a certain level. If it still happens with a new receiver pack, then I would borrow another servo and plug it in to the steering channel first, then throttle to see if you can get the problem to repeat. If you already have a new receiver, then I doubt it's the problem. Heck, it could even be your switch. You just have to go through a process of elimination.