Intermittant sim error on ipad2

I have an iPad2 (MC774B) 32GB WI-FI 3G. OS 6.0.1

I have had a Three Mobile data sim installed and have been having intermittent problems. The iPad will lose the data connection and display NO SIM CARD INSTALLED, this can usually be rectified by switching to airplaine mode and back again. recently the frequency and severity of this issue has increased, I have just experianced 5 days without 3G conectivity. I have tried: resetting connection settings, powercycling, removal and cleaning of SIM card. etc but the problem persists.

Please help

Sam