GRC + USRP2 + WBX Duplex problems

I have been having a struggle with the USRP2 and GRC owing to the
limited documentation. The USRP2 documentation is really sparse. However
I have finally managed to get some basic examples working. Hopefully it
is a short step from there to get what I really want which is to
implement a GMSK modem with FEC for our VHF/UHF satellite ground
station.

So, Alex OZ9AEC came to the rescue with his examples pages

Based on this:

I can receive WBFM - and other modes as audio
I can transmit WBFM & other modes from a .wav file

What I can not do is both at the same time, i.e. duplex. This is using a
fast new system with a Core I5 and Gbit interfaces running Ubuntu linux
and the latest GRC released version.

The problem is whenever I try and do TX and RX at the same time the RX
stops working completely. I am clearly missing something vital here but
there is no manual to tell me what it is so I am hoping someone else had
solved this and can advise.

Another point. GRC claims the USRP2 sink is 100Ms/s. The USRP2 manual
claims the DAC is 400Ms/s. This cost me many hours day failing to
understand why my examples would not work.

Mike