The hitec digitals sometimes need a y harness w/ built in choke/amplifier (Available through MPI) when used with futaba reciever (especially pcm- lower signal voltage) nothing wrong with them, they just need maintained signal voltage, and long leads and y's dont help. the choke/amp should solve your problem. Also, always watch your battery voltage - with digi's the more mah the better (easier to stay on the safe side) It has more to do with the servo not 'hearing' the signal than just getting a little tired.

hope this helps,
Ted