Blackberry still rebooting after fixing

My phone kept rebooting on its own, would switch off and reboot on its own while in the middle of taking out photos with the flash on and would freeze while using it. I took the phone to Vodacom Telepgae - Port Shepstone. The phone was sent way for 2 weeks. When i went in to collect the phone, they said they had given me a new phone because they could not fix it. It was not a new phone, but rather a refurbished phone which is now giving me the same problem. I took the refurbished phone for a software upgrade and it is now worse then it was before the upgrade. The phone keeps rebooting on its own every few minutes, no matter what I'm using it for, whether taking out photos, making a call or even using apps such as bbm and Facebook. I am paying for a phone that i cannot use without it giving me endless problems from the time i took out my contract. It is ridiculous that i am paying monthly for a faulty phone, and Vodacom's customer service is so horrible that no one is willing to help. I am seriously rethinking renewing my contract with Vodacom when the time comes. I need someone to call me with solutions to this reoccurring problem.