self lock

[john_q]

The phone locks itself while doing something, it doesn't matter what it is as it happens randomly.
Also sometimes the wallpaper goes black and I would have to manually change it back to what you it was, it's one of the default wallpapers from Sony. Could it be a software bug or a hardware failure?

[Thommo]

Possibly software so you could try booting into Safemode or performing a system repair

Performing a system repair using PC Companion as this will often fix issues - This will factory reset your phone and erase all data

Switch off your phone and unplug from Pc (Hold volume up and power for around 10 seconds)
Start PC Companion and select Support zone then Update my phone/tablet then in Blue Repair my phone/tablet and follow the on screen instructions - When prompted connect your phone still switched off holding volume down or back button - This should start the repair or reformat process

If you are using Windows 8/8.1 or a 64bit operating system then adjust the settings for PC Companion and run in compatibility mode and chose Windows 7 or XP

[Uliwooly]

Does your phone has a protective case or film that might be blocking the proximity sensor?

Do you have an antivirus or taskiller?

[john_q]

No anti virus proximity sensor is only for when it's in call shouldn't affectthe wallpaper or lock

[john_q]

I would try that and see what happens

[Phy]

Does the same happen when you boot on safe mode?

http://talk.sonymobile.com/t5/FAQ/How-to-boot-your-phone-into-safe-mode/m-p/348008#U348008

[john_q]

Haven't checked yet will do soon

[john_q]

Unfortunately self lock is still happening with a lot of rebootingalso happening, the rebooting seems like it happens 2-3 times before it calms down and I'm able to use it, but I have never heard of a smartphone rebooting several times in quick succession. After the repair through pc companion the first time it self locked was when I was doing the initial setup so it can't be an app which I downloaded and plus I only have like 3-4 apps downloaded to the device since. I wonder whether this is a hardware problem

[Uliwooly]

Repair the phone with PCC > do NOT install any apps, uninstall any antivirus app 

PC Companion (PCC)

Bridge (for Mac)

Alternatives on How to backup Xperias

http://talk.sonymobile.com/thread/36355

[john_q]

That is exactly what I did. I repaired the phone and during the initial setup before I have any chance to install any app it locked itself. After the repair the loss of wallpaper seem to have disappeared but it reboots a couple times a day. I don't use antivius because if you know what you install it is impossible for the phone to get affected by anything.