asterisk users - Mar 2006 - misdn <--> zap problem

I've got a problem with chan-misdn

I'm using asterisk with a hfcsusb-adapter in nt-mode connected to an 
isdn-telephone
making calls to other internal clients like sip or sccp are without 
problems
if I call into (or receive a call from) the pstn via a zap-channel (Digium 
E1-card) my outgoing audio from the misdn-device is very choppy.

is where anything I can do about it ?