i don't think network issues will cause fps lag...

if it drops on your every 1st battle once u start diablo 3...it's normal...
if it persist after a few battles...then it's a problem and try to turn off all nt so important applications/software....
if still not working smoothly, then sorry to tell u i dunno how to fix it...can try to see got any solution on the b.net forum...good luck