Strange Errors from GPS/GPRS SIM908 with Aruduino MEGA 2560

We are having problems with the board that are hard to reproduce, so I
thought I will describe the circumstances and hopefully you can
suggest a way to fix it, or a way to gather more information.

We're using the shield with an arduino Mega 2560. It's being powered
from an AC-to-DC converter at 7.7 volts. We've measured Vin on the
shield at 7 volts.

We're using the code hosted here:

http://www.cooking-hacks.com/documentat ... spberry-pi

In the section "Realtime geolocation tracking". We have modified it a
bit, but not much.

Most recently, we've been having problems with the section of the code
that establishes a GPRS connection to upload the GPS coordinates to a
webserver. When the arduino sends this command:

AT+SAPBR=3,1,"Contype","GPRS'

The shield returns the following:

AT+SAPBR=3,1,"Contype","GPRS"


SIM900 R11.0


ERROR




This behavior is intermittent. Sometimes it works, then a day later it
doesn't. We can't figure out what the difference is, because we're not
changing hardware or software. We also sometimes have problems with
these commands:

AT+SAPBR=1,1
AT+HTTPINIT


Sometimes, the failure is because the shield returned "ERROR". Other
times, the shield returns nonsense strings (i.e. ÿÿÿÿÿÿÿÿÿÿ).

Any ideas about what could be causing these failures?