Facing similar issue.
After updating to 4.3 on Samsung Galaxy S4 I9500.
Wifi works for a a few minutes. Then stops working "Your internet connection is unstable"
Auto switch setting is unchecked.
There are issues with the new 4.3 update and WiFi for some reason.
My GS4 doesn't automatically reconnect to known WiFi networks.
I have to either turn off and on the WiFi or the airplane mode!
Not a big deal but it gets a little annoying sometimes. :banghead: