Unhappy [Q] Xperia Neo MT15i restarts in sleep mode

Hi all,
I recently buy a xperia neo mt15i and take to a shop to unlock.
Then I start with bugs, every time I press the power button to go to sleep, it restarts the phone.
I install a app to lock the screen, and happens the same.
I flash the 4.0.4 android hoping to solve the problem but no, recently every time I make a call or answer after maybe 5 secs, it dropped.
I flash different roms, that I find for my phone, but in vain.
I buy a new battery, I put a paper in the battery, etc. but its on the same.
Is there anything that can be done, or forget it and get a new phone?
Thank for all the help, in advance.