I've been having some issues with bitcoin mining lately. I got gui miner tp work and my 5850 is cranking out 275Mhash's atm.
Whenever I launch Guiminer I look under teh summary and my 5850 is going on and off wit hconnection problems.
If your router/modem has latency issues (spikes), that may be the culprit.
I don't think it does. But it probably does knowing caomcast :/