Default Weird Wi-Fi problem

My rooted International N9005 LTE, stock rom, build number KOT49H.N9005DXUENE4, baseband N9005DXUENE3, has a weird problem never experienced on my phone before.

After I connect to ANY wi-fi, I have just seconds before "your wifi connection is unstable" message. Sometimes I can download/open web with it, until I simply can't. Then I reboot my phone, and I am able to surf the web once again for a limited time. O.o

Any ideas what the heck/wtf/what the hell is going on here and how to fix this?