Windows 7 - 64bit
AMD 13.1 & 13.4 drivers. (And whatever sdk is with those)
Cgminer 3.1.1
What else is running? I have 2 machines with the same setup with 4x7970 with no problems.
I suspect this is related to some reconnection or IP change or something along those lines so you need a flaky connection to have a problem.