It seems to be that this instability is not just caused by drivers but UPnP being apparently overwhelmed back at the router by something that must newly be going on in the Windows 8.1 networking stack and/or Intel drivers since the (universally acclaimed to be stable) driver version 14.8.8.75.
Woody over at http://woodygems.com has done some testing and has discovered that the UPnP implementation on some routers/firmware seems to be the actual problem (not the driver) and has identified 3 routers so far which, when UPnP is turned off, this symptom goes away.
[I note that Woody has also discovered that Intel Centrino WiFi does NOT drop out (whatever the driver version) when connected to a Netgear DG834G]