This happened fairly often a couple years ago, and then it gradually went away. I never saw anything that definitively identified the cause of the problem, and there are unfortunately several different things in the path that can cause it.
In the SIP world, there is the periodic registration with the SIP server, but GV doesn't use SIP, so ignore any of those settings.
If you have a problem with your home internet service, it could cause GV to erroneously detect that the call has been hung up, so it kills the connection. Happening at exact intervals is what is so mysterious. So many other factors can interfere. Just one that comes to mind is something else that gets kicked off periodically on your home network, like one of your devices periodically contacting a server to sync files or download updates. If something even momentarily swamps your network connection, it could cause the drop.
For now, you may want to try running the VoIP call quality test at this website provided by Visualware. The regular
speedtest.net tests are meaningless for VoIP, which is sensitive to network jitter and latency and brief dropouts, not shown by basic speed tests. Pick your geographic location, pick an endpoint, and let the test run a simulated VoIP conversation. Use the G.711 codec. A MOS of less than 4.0 is a sign that you have network problems.
http://myspeed.visualware.com/index.phpIf you do have call quality or reliability issues with a GV call, note that when you immediately call back, Google's system is keeping track of these events. If it determines a pattern of repeated dropped calls, followed by call-backs, it flags the call route for investigation.