Thanks for getting back to me Mike.

Nope, no extensions. I've tried wiggling the plugs a bit, checked for any obvious damage and I can't find anything visibly wrong with either servo.

Beats the tar out of me what's going on but it would seem to suggest some sort of connection problem, or a problem with the boards in the servos. I just can't see two of them doing almost exactly the same thing and as I said, the programmer works just as advertised on the other servos I've tried it with.