W580i — Phone gets hang, auto switch off frequently

Dear Customer care,
I had purchased W580i cell from mumbai last year in Feb. But since then it is recurrently giving me the following problems - phone gets hung frequently, gets switched off automatically while operating, cannot hear the voice of other person or thier ringtone while making a call. I have upgraded the software 3 times, but still I am facing the same problems. Everytime i go the the service centre, i am being told that after upgrading the software the problem might get solved. First i went to Dadar Sony ericsson authorized service centre and later on twice (in December 08 and Feb 09) to sakinaka authorized service centre.
This gets irritating, because most of the times during an urgent call, such issues make things worst.
Can anyone please help me with this regard. How come after so many upgradations, the phone has the same problem??. Has Sony Ericsson stopped manufacturing good quality phones?? I feel cheated and dejected for buying this phone and would like a concreate answer. Is any one listening..?? I would like to have a replacement.