We would have to see a complete diagram of your situation to be sure of course, But it sounds like a typical home where you need to tie the grounds together somehow. The best way to do that, If possible is outdoors, Underground with bare copper wire, At least #6 gauge or heavier. Adding ground rods every 16 or so feet along the path.http://members.cox.net/pc-usa/station/ground0.htm
A long read, Be sure to also go through all the links he provides.
Also scroll down to the installation instructions on this one:http://www.comm-omni.com/polyprod2.htm