It sounds like your servo is pulling some major amps and it is dropping the voltage to where it will cut off the servo/pt.

I had the same problem with my spektrum/ko servo...this is what i did to fix it and i have never had a problem since:

I used a servo wire that i soldered to a novak capacitor (just the positive and negative obviously) and plug that servo wire connector - that is only connected to the capacitor - it looks funny, but you don't have to solder a capactor to anything but the connector you use to put in one of the extra connections in the receiver (if your receiver does not have enough plug-ins, just buy a "y-style" adapter - this is what i had to do)

What is happening it sounds like is the voltage/amps gets high enough from the servo, which then pulls the voltage down in the receiver, then the receiver goes into failsafe mode. What a capacitor will do is "store" a little voltage/amps/power so it keeps the power at a nice steady level - and your spektrum has enough power.

As you can tell, I am not electronically oriented, but i can e-mail you a pic of what i did - super easy fix and it should cost you no more than like $5-$7 and take no more than 5-10 minutes

If you look on the spektrum thread here, you will find a little better explanation than the lame layman version i just gave.

mcgoo