USRP2. simultaneous sending and receiving

I tried to send out a sine signal to a Oscilloscope while simultaneously
receiving a sine signal from a signal generator.
When I’m doing one at a time it works fine, but if I do both together
the amplitude of the received signal drops after a really short while to
almost 0.

I’m using an USRP2 with RFX900 Daughterboard. I flashed the USRP2 with
the newest firmware.

The USRP2 is connected to a Thinkpad T41 with a Intel Corporation
82540EP gigabit ethernet controller. The Thinkpad has a 1.4GHz processor
and 768MB RAM. It runs debian testing on it.
I use the developement version of gnuradio (3.3git).

The code is nothing fancy. The Block Diagram looks like that:

| signal generator| -> | Float to Complex| -> | USRP sink |

| USRP source |-> | Complex to Magnitude | -> | File sink |

Does anyone has a suggestion what might be the problem and how to solve
Cheers, Pascal