Message “popping from Tx, samples popped 208/680” means that TX thread was not able to get requested number of samples from TX FIFO before timing out.
Your data rate is too low. Have you changed sample rate? Do you use USB?
I have checked dualRXTX code and found a bug that was causing stats to be printed too often when GNU plot is not used. That may cause some slowdown, but it should not be as extreme as in your case.
Do you get any errors before streaming starts? Normal output of release version should look like: