I had this issue too. Unfortunately for me, I had the handset bought in UAE while i reside in India. I had it sent back to the customer service center in UAE and all that they did to fix it was to update the phone software. I have no idea what else they did. I was not given a replacement though!
My problems had started after I had flashed the global version of firmware followed by UK version and finally the leaked PR 1.2. The disconnection finally stayed on even after getting a new SIM.