Phone repeatedly boots (restarts by itself) when kept on hard surface

I have got a Xperia SP C5302 which began to show problems from yesterday. It is repeatedly getting restarted by itself whenever I am placing it on any kind of hard surface(ex: table) but works fine when I am holding it in hand or keeping it on top of mattress. **What I have tried -** 1. Did factory rest couple of times but nothing fruitful. 2. Did a software repair through Sony PC Companion but it didn't work. 3. Tried googling in and around but no good result as of now. Phone is running stock rom **12.1.A.1.205** **Android – 4.3 Jelly Bean** Let me know if any further information is required from my side.