1. My problem: The problem that I face is that when I try to connect to a wifi network one out of the two following things happen:
-It connects and instantly disconnects
-It connects and after some time losses connection and says that it's out of range. This can happen after 5 seconds or 15 minutes/it's totally random.
I think that the first thing is connected to the second so let's just discuss the second.
2. Where and when: It happens everywhere and always. It happens at my home, at my school, even in Greece where I spend my vacations (just giving you some examples.).
3. Things I tried to fix my problem myself: I googled my problem for over 1 month now and found a lot of those "standard wifi fixes" I mentioned before. I checked all the settings on my phone for example avoid bad connections, disable wifi during sleep, battery saving etc. I did a factory reset 3 times, I even screwed with my router (even though it can't be my routers fault as it happens everywhere as stated and I am fairly educated in networking and setting up a router). And yes, I did update my phone to the latest version of android available which is 4.1 for me.
4. Contacting my operator: I contacted my operator and gave them a brief overview of my problem. So I went to the store and they sent it to the repair center/company that fixes Sony phones in my country (note: my phone is still under guarantee). After 2 weeks I got my phone back but they wrote that they didn't find the problem and just did a factory reset. After 1 day of usage, with the of course not repaired phone, I sent an very detailed email to the repair center explaining my problem and they told me to send it back again which I did. After a couple of days passed the repair center called me and the guy who was supposed to fix my phone told me that he did a factory reset yet again and that he isn't finding any problems with browsing the net with the phone.
5. Some important information: The phone had never been exposed to any water/fall damage in any kind of form, it was neither rooted/flashed or anything like that and there were no applications that could have messed up any files or settings (for example wifi fix apps and such). I am running the latest available firmware (checked with xperia update checker, sus and PC companion) which is 9.1.A.1.141 R1.
6. Some probably not so important information: A friend of mine which I meet quite often has the same phone but has an different operator is running 4.3 and has no problems with the phone what so ever. Also when I went to the center of my operator the guy that helped me tried to connect to the router he was literary 2 m away from and it instantly disconnected him saying "not in range".
7. Don't say those things as I already tried them without success:
-everything connected to wifi/network/battery saving settings on the phone (avoid bad connections, auto IP, Static/DHCP etc.)
-my router settings (channels, security etc.) - every other device works perfectly fine
-SIM card - calling, mobile data and such works fine
-apps - never had "wifi fix", battery saving or any kind of form of apps on the phone that would "improve" it's performance
So I am really desperate right now and would really appreciate if someone could help me.
EDIT: I forgot to mention that after some time after a couple of minutes it shows the network as being available again (but doesn't reconnect) or waits until I restart the phone before it let's me connect to it again.