Hi Raghu & Vikas, I own a Samsung Galaxy Y too and faced the same problem over 3 weeks until last night, I had contacted Samsung and researched internet everyday to find a solution without landing myself anywhere, finally I thought of doing something on my own as this thing was killing me. I do not know if this is completely fool proof but haven't faced the restart problem since then.
Please follow the below steps to have this sorted out:
Go to Settings ---> Click on Applications ---> Click on Manage Applications ---> Click on All tab ---> Select each and every application you have on your phone and clear cache.
I guess the reason behind this is Galaxy's RAM is only 290 MB & Phone Memory is 190 MB and if we do not clear the cache and stuff the memory gets piled up too much for the phone to handle.
As I said, I just got this last night so hoping this is the main cause of the restarting problem.
If this does not work will try and find something else for ourselves.
Thanks & Regards,