MC35i siemens embedded modem error

Hi, all,
I am testing on my own MC35i GSM/GPRS embedded modem pcb. It is working from power up meaning that I can send and receive sms through the modem. But after minutes (sometimes hours), it just hang there with the led indication same as 'initialization'. But it returns 'sim not inserted' error when i tried to send an sms. I have checked the schematics and PCB layout, but can not spot any mistake.
Has anyone encounter same situation b4?
Thanks.

Wanchope

In the datasheet
bergmann-electronics.com/downloads/mc35ihdv01021070475.pdf
Page 45 under Sim interface section, saying that "If the SIM card initialising has been done, but the card is no more operational or
removed, then the module will never search again for a SIM card and only emergency calls can be made." My board seems enter this state. The engine would not search for the network again as it regarded the sim card spoiled not removed. But actually my sim still in and operational. (i tried others sim cards anyway, same thing happen). And i also using the molex holder, everything seems fine. but just do not know why the engine thought the sim card was not there.
Any idea, friend?

wanchope