Voted too.

But IMHO its not related to SIM or Phone settings because the first day I got it I used it without sim, only connected to WLan and it used to reboot frequently.
Then I tried with an H3G SIM and used to crash too.

Actually I'm using Wind SIM and still crashing.

The only things that changes frequency of crashes is MfE for me. But never reached complete stability as it reboots even when display is just about to light off and no apps in background.