USRP and Variable Bitrate input

Hello all.

I am slowly making progress on a streaming application but am coming
across some problems with underruns on the transmit side.

I am using a C++ program to modulate an input signal. Unfortunately the
input signal to the C++ has a variable bitrate. The problem is that the
C++ program writes packets once enough data is filled and then sends to
a GRC script which runs the USRP. When the input to the C++ program is
somewhat stable, I see no underruns on the USRP, but when the bitrate
changes a bit, I see lots of underruns which is causing problems on the
receive side because the signal has lots of interruptions.

Does anyone know of any tricks I might be able to try in GRC? I don’t
know if there is some way to manipulate a stream like the one I have and
make it appear more constant to the USRP.

Unfortunately big changes to the C++ program are difficult for me to do.
Thanks - Tom