Sony Xperia U mobile handset repeated faults

Location/place: Vadodara

Name of company/service: Sony Mobile Handsets

I had bought one Sony Experia U(ST25i) on 250612. Battery and processor hangup problems came within 2-3 days of purchase and after some time I was forced to handover the same to the co’s Vadodara service center vide workorder no. SE31212012148. After several follow-ups I got a SWAP(old used and repaired in return of my brand new piece) instrument back on 16082012. The same is again faulty and is again handed over to the same service center vide work order no. SE31212012441 on 270812. Again I have been asked to wait for ten days.

Now my submission as follows:-

My new instrument was giving problem and you have given me a used and repaired instrument after 17 days. Why not a new one?
When the co is taking back a new instrument why not giving a standby instrument or a choice to the customer to get back his money?
Twice faulty instruments have been handed over to me. What are the chances of me being third time lucky?
What will be the warranty date now?
Since I have already raised all these questions with Sony without any reply which authority shall I approach now with my grievance?