Why does my phone shut off, start up then vibrate then freeze?

I have had my phone for about 8 months and the last couple of months it has been playing up.

Took it to the TelstraShop and he suggested to download the update for it. So I did that and the phone worked well for about 1 month.

NOW the thing shuts off, usually when I am in the middle of something (naturally) then restarts and vibrates 7 times then...................nothing, it freezes on the blue Telstra start up screen.

Any suggestions?

I have had a look and can not see any new updates. I have also reset the phone back to factory setting (several times) and it is still shutting off.

HELP before I smash the $%!* thing!! Never had this dama with my old trusty NOKIA.

Cheers in advance

M