I had the EXACT thing happen when I was using Hitec servos with my spektrum reciever.. (the 525's).. A hitec rep told me they were difected, so I sent both of them back to them, they tested them and said they were fine. So I tried installing them again. Same thing. Tried two different brands of servos and they worked fine.. I think with some recievers, they just dont work. Maybe this is the case with yours, it sounds exactly what was happening with mine.. Hitec told me they should work fine, but I could never get them to cooperate.