USRP2 Newbie

I’m trying to get a FM audio transmission around 100MHz from the USRP 2
with the Basic TX board but the output signal when viewed on spec
analyser is not constant and seems to sweep and not be stable. Have
tried the examples supplied with no success at generating meaningful
output from the USRP2, to debug this I’ve fed the output from the WBFM
encoder module back into a WBFM decoder which works so I’m wondering if
I’ve got the correct USRP2 settings and have some questions below:
The Basic TX board has 2 outputs, when fed with a complex signal does
that mean that TX A output has the I and TX B the Q signal ? The D/As
are rated at 100MS/S but with interpolation can output 400MS/S, which I
assume means it can generate a maximum 200MHz signal (nyquist rate)
What is the purpose of the USRP sink frequency ? does this feed the
digital up converter which it mixes with the baseband input signal ?
In my example if I have an audio signal (samp rate 32KHz) fed into a
WBFM (with same samp rate and quadrate at 320K) what should I set the
interpolation rate to on the USRP to ? I though this should be
100Ms/320K which is 312.5, which doesn’t seem to fix the problem.
Am using latest GIT build of Gnu Radio with latest FPGA and firmware
(non UHD). I’m also a newbie so I’m on a steep learning curve so
grateful for any help.