xFi router drops wired ethernet connection

Hi All - xFi works on wifi, no problem.  I have an HP switch/dumb - and I was using Comcast Business - and never had a problem.  Now when I add more than 2 ports to the switch, the router drops the wired connection completely.  Switched to different ports.  Switched out router as well.  I feel that this must be a software issue.  I have tried to vary what I connect.  I am using HP 24x1Gb switches, Ubiquiti Wifi, and have Sonos and Tivo connected as well.  But when I test, I just connect my Desktop and then my wife's desktop - that works.  Then one more - and I have tried varying what I plug in - and the whole thing collapses.  Help.  I am network savvy but can't figure this out.