Hello Patpin,
I have the same problems with a Mega 2560 and the GSM shield. It's really weird. I see also corrupted data at the serial output. On the Uno everything works fine.
I have used different RX pins like: 11, 12, 50, 51 etc. But no good result.
I have also used an external power regulated between 7 -12 Volt. But no good result.
Do you know why you can use the interrupts written at
http://arduino.cc/en/Reference/SoftwareSerial"Not all pins on the Mega and Mega 2560 support change interrupts, so only the following can be used for RX: 10, 11, 12, 13, 14, 15, 50, 51, 52, 53, A8 (62), A9 (63), A10 (64), A11 (65), A12 (66), A13 (67), A14 (68), A15 (69)"
and not the hardware interupts written at
http://arduino.cc/en/Main/ArduinoBoardMega2560"External Interrupts: 2 (interrupt 0), 3 (interrupt 1), 18 (interrupt 5), 19 (interrupt 4), 20 (interrupt 3), and 21 (interrupt 2)"
I'm not really understand why I can use the change interrupts. What is special about these I/O?
I have also used the newest IDE 1.0.6. Then my sketch even don't compile a good code for the processor. With the new IDE 1.0.6. I become a lot of errors.
Could it be timingproblems?
Please who can help us with the same problem.