Thanks Aishwarya
I have installed many apps like twitter,opera,android assistant etc.Also took the net connection.Problems started then.Then I went to samsung service centre and they told it might be a prob with s/w.So they do master reset and gave it back to me.It works well when I am in my home.But it causes problem at my workplace(less range for idea there)
Then I installed android on/off.For the past 2 days it works fine.I also planned to move to airtel as it has better range here.