OK, you're getting close. The next step is to remove the extensions and 1 servo. Take 1 servo and pul it directly into the receiver. IF that works ok add the second servo direct.

I'm betting that one of the two servos will be the cause of the glitch and not the extensions, however if both sevos work OK when plugged in directly then the problem must be in the extensions. Again, add them back in 1 at a time.

Also, how are you connecting them. 2 channels on the RX or 1? If you use a single channel for 2 servos there must be a Y cable in there somewhere, have you checked that? If you use 2 channels the problem could be in 1 of the receiver channels, try it with a Y instead.

TF