Frequency drift jumps between two USRPs

[xarteaga]

Hello, I am relatively new in NI USRP and labview.

I am trying to do a labview program which is able to send a message from one USRP (2920 model) to other one. I programmed it using labview 2012.

I programmed a quadricorrelator for measure the frequency offset.

I programmed another VI which tests my quadricorrelator. This VI measures the frequency using the quadricorrelator and also a NI Phase-Lock Loop (PLL). After a while I get the following:

The measured frequency sometimes jumps from one frequency to another one. I have no explanation. The USRPs are connected each other using the included cable and the attenuator. The ports used are the TX1 and the RX1. 

 Why is occurring that? Have I configured properly my pair of USRP?

I attached the VI I used.

Thanks in advance,

Xavier