I have a Sagem MW3020 which does all it is supposed to do with the sim card out, but when I put the card in, it powers up OK, and connects to the network, and then after about 20 seconds it freezes. It will come back online again after sitting for quite a few minutes, but will then freeze again after about 20 seconds. I have checked fields '0' and '191' and they seem to read what they are supposed to read. Any help would be most appreciated.