Phone is automatally going switch off - Nokia 7210

There could be a problem with the phone's battery.
Try this.
Take the rear cover of the phone off.
Take the battery out and clean it with a soft dry cloth.
Clean the inside of the cell where battery is to be placed.
Check that golden connection where battery connects with the phone.
Replace the battery and switch the phone on.

If the problem presists try updating your phone's software from
http://www.nokia.co.in/support/product_support/nokia-7210-supernova