The ring tones & sounds for my Loud, Medium & Normal profiles come & go on my Blackberry 9900. The vibration feature still works when the sounds/ring tones don’t. Re-booting brings them back – what gives?

I hv bought a new BB 9900 and since have been having a problem of ‘Ringer going Off’. The ringer goes off automatically anytime and I end up with missed calls, I cannot make calls in this condition of the BB ie I make a call and I cannot hear the ringing nor can I hear the other person. I have to do the Battery Pull and then the BB starts again, for sometime, maybe a few calls, then again the same cycle repeats. Tried updating software, tried sending it to the BB centre for checking, still problem remains, presently on software version 7.0 bundle 2395