My Samsung Galaxy Tab S, running KitKat, has been having problems over the last few weeks, where the WiFi connection constantly drops out and reconnects, causing connection issues on loads of sites and apps. It's been happening at home on a new router, and at work on some fancy new access points that mesh together to bathe our whole office, so I'd put it down to the phone (I use the 8.4 inch tablet as a phone anyway (which looks pretty comical)) itself.
I finally decided to dig into why, and found this forum post, where people throw around various ideas like Samsung SideSync being the cause, though in my case it was much simpler.
I'd turned on Smart Network Switch, under the WiFi settings, which claims to maintain a stable connection by switching between 4g and WiFi when needed. There's something wrong with it though, because it was saying that a WiFi connection is unstable even when it's got full bars in an unpolluted area, so it was just causing lots of unnecessary switching to 4g. Since I turned it off, the WiFi connection has been reliable and stable, rather than switching several times per minute.
Problem solved, and I hope it helps someone else out there.