Anyone has this phone model and face this problem before?

It keeps shutting down and then restarting itself..now can't
even call out without it restarting itself every few minutes...and
of course that caused the phone call to be cut off...damn
annoying...
Anyone knows where can I go to repair this? I got this phone for
free from Starhub but I couldn't find my warranty card and original
receipt for the phone. How much to repair?
Sigh I have only used this phone for less than 6 mths...