Laptop Suddenly shutoff...dell latitude e5400

hi Dell, I have dell Latitude e5400 and for last 1 month I am facing a problem, my laptop suddenly turns off automatically withoutay error, it happens everytime when I am using specially on Wfii and when I take my system to a technician then it works proper. Never turns off whenever I go to the concern person. System technician checked ma and charged me lott $$ but after 1 day working its doing same thing . please help me out with this if any body know about the problem....