How to stop FFT from going uOuOuOuO ?

The uO means that the USRP Rx path is overrunning.  That is, it's not being serviced in time and you have gaps in your data.I suggest you try a higher decimation rate. 

** Every time you see uU the USRP has nothing to transmit when it needs data, so it sends zeros. 

** The "u" stands for USRP, and the "U" for "underrun" meaning your PC is not generating data fast enough to keep the USRP running.  Sometimes you may also see uO, meaning USRP overrun.  This happens on receive applications, where your PC is not reading out the data FROM the USRP fast enough, and you end up dropping data because your buffers aren't big enough.  The same things apply with aU and aO, but for audio devices.