Same problem here!
I am running gnuradio from trunk (rev. 10496), on kubuntu 8.10.
I have a USRP2, firmware and FPGA updated to the last available version.
I am also a very beginner.
If I set any frequency other than 2.4GHz (by just modifying it in the
grc block) I get a choppy, blinking, and buggy signal.
There are so few examples on the USRP2 in the code and it is not so easy
to understand how to work with it if you have never used a USRP1.