Hi Pierre ,
It has couple cases to have this happening .
1, Module reboot itself .
Registration process would consume some power , if power supply is not enough . module will power off / On periodically . In this case pls check VBAT pin of BG96 module if there is voltage drop .
2, AT+COPS=？ was used to scan the network , in this case module will increase tx power to talk with cells . If power supply is not enough , reboot would happen.
or early firmware of BG96 may have this bug with AT+COPS command . pls send the IMEI , then we can find what firmware it is .
3, Module was reboot by MCU , while some ERROR returns , MCU will reset the module . So pls check if there is ERROR happens after AT command .
4, There is another case , module it is in PSM power saving mode . Pls check AT+CPSMS? if it is 0