Hi Marcus M. and Marcus L.,
Thanks for the help. Good point about this being more of a USRP digest question. Thanks for referring this to that as well. And unfortunately, that was the correct assumption that I did not use a torque-controlled wrench to connect them either, which maybe I could look into for some improvement.
These are USRP N210s, with SBX daughterboards rev 5.1. I am transmitting a single tone (GR_COS_WAVE in grc) at 915 MHz. I do this for all of the USRPs. I figured some variability is expected, as you mentioned, but perhaps a range of variability might be informative. Hope that helps.