I've had the same problem, but was only using two computers. It was a
ground loop. Simple take a heavy gauge wire (12 in my case) and ground
each computer chassie to eachother then ground that to the Stridsberg
multicoupler. In my case I didn't need to physically ground the cases
of the scanners as that was already taken care of my the outer sheld of
the coax between the scanner & stridsberg.

Someone mentioned terminating the unused Stridsberg output. I have 2 of
the amplified 4 port multicouplers and 1 of the 8 port amplified
couplers and never terminated unused ports so in my case that wasn't an
issue.

George - Daytona Beach, FL - KI4FIA
Look Me Up On IRPL Node 4053 or Echo Link 50666
http://www.MilAirComms.com