My mobile not working properly

On dt. 19/06/2015 my xperia z1 got dead on of sudden though I have charged it full. I reset it as per sony guidlines. Then I shown it to service centre. Person telling me that you have to replace mobile by paying half its cost. I refused to do it. After 3 days I have agian plug in my mobile and it get switch ON. But again it get tripped though battery was 65% charged. This is happening repeatedly. Please suggest me the solution. Mobile warranty period already over in Nov 14.