Thanks for reaching out, all input is appreciated.
The file in use is from files.ettus.com/tutorials/Lab_5.grc.
The "gross" mismatch in sample rates comes about when you set the Rational
resampler output rate to be exactly what the UHD:USRP Sink is expecting ...
in this case 250K.
The incidence
If you're getting long strings of 'U', this means a *gross* sample-rate
mis-match somewhere in your flow-graph,
rather than the more-subtle "two clock" problem.
Make sure that everything agrees internal to the graph about
sample-rates, and, perhaps more importantly, that your audio *hardware*
