Connecting to GSM/GPRS unstable, eventually hangs

Hardware Arduino GSM Shield
4

OK, I figured it out. Following Code hangs in interrupt when there is poor GSM signal - Arduino GSM Shield - Arduino Forum, it seems the problem occurs when starting communication with the GSM module and a reply is never recieved. So, to fix the stuck on AT%13% bug, simply change this in GSM3ShieldV1AccessProvider.cpp:

GSM3_NetworkStatus_t GSM3ShieldV1AccessProvider::begin(char* pin, bool restart, bool synchronous)
{	
	pinMode(__RESETPIN__, OUTPUT);

	// If asked for modem restart, restart
	if (restart) 
		HWrestart();
	else 
 		HWstart();
  
	theGSM3ShieldV1ModemCore.gss.begin(9600);
	// Launch modem configuration commands
	ModemConfiguration(pin);
	// If synchronous, wait till ModemConfiguration is over
	if(synchronous)
	{
		// if we shorten this delay, the command fails
		while(ready()==0) 
			delay(1000); 
	}
	return getStatus();
}

To this:

GSM3_NetworkStatus_t GSM3ShieldV1AccessProvider::begin(char* pin, bool restart, bool synchronous)
{	
	Serial.println("gsm.begin()");
	pinMode(__RESETPIN__, OUTPUT);

	// If asked for modem restart, restart
	if (restart)
	{
		HWrestart();
	}
	else
	{
 		HWstart();
	}	

	unsigned loopCnt = 30;
	theGSM3ShieldV1ModemCore.gss.begin(9600);
	// Launch modem configuration commands
	ModemConfiguration(pin);
	// If synchronous, wait till ModemConfiguration is over
	if(synchronous)
	{
		// if we shorten this delay, the command fails
		while(ready()==0 && loopCnt--)
		{
			delay(1000);
			Serial.println("gsm.begin() waiting for ready");
		}			
	}
	return getStatus();
}

This way, even if the hang occurs, it is timed out properly.