VoIPBuster or IAX problem with moh

Okay, here is the problem...

Using Asterisk@Home 1.3

I make a call with VoIPBuster (My only IAX route at the moment, maybe this
is significant?)

Normal calls are fine, however, if I put the caller on hold, the music
sounds very choppy to the caller, it seems to be ok if the caller makes
noise all the time.

Is this my Asterisk box (I guess so) or VoIPBuster?

If it is my Asterisk box, what can I do to improve it, as it sounds crap!

If I call out via Sipgate (SIP) (or receive a call via Sipgate) it is all
fine,

Thanks for any hints!

Sparks...