Tablet Z restarting no matter what stock ROM has

So I had this SGP321 for over a year now.Bought it brand new from this guy on the internet for a suspicious low price.Not a real shop or anything so no waranty. It worked very well for a few months then I noticed it started rebooting every now and then.After a OS update that was rolling out at that time (can't remember the version) the reboots were more often.After the KitKat update arrived and I downloaded it,after the process was over and the phone had to reboot,it entered a reboot loop that could not be stoppped (apart from forced shut down,but with same thing when trying to turn back on). I repaired the OS with the PC tool,no changes.I flashed older versions of stock OS with flashtool,reboots every couple of minutes. Then I installed Cyanogenmod 11.That worked very well,no reboots,but had many bugs and battery draining fast in stand by (about 1% per hour no matter what I deactivated,or with various apps to induce deep sleep). The new CM12.1 does not work,keeps rebooting.

Now,yesterday I tried again and again to get the tablet to work with a stock ROM. I only intend to use it for reading comic books,so all I want is a stable ROM with good battery life.So I would like to have a stock one. But why does a custom one run fine on it and a stock one keeps rebooting? I tried Sony forum but they were at a complete loss and sent me here for help.Any ideas?