Underruns at medium sample rates using UHD

Hey list,

I regularly encounter underrun messages when running the UHD transmitter
(uhd_mimo_sink or uhd_simple_sink) with sample rates of 12.5 MSps or
higher. The OS (Ubuntu 10.04, 64bit) setup is according to the
guidelines (realtime priority for user, buffer size set to recommended
50 MB) and the current git version is in use.
While runnning the application the system load using a Quadcore CPU @
2.66 GHz and 4 GB RAM is moderate but never high.

Has anybody else experienced such underrun problems and/or knows a
solution how to solve it?

BR Christoph