GPRS Shield: Problem with data > 1000 byte

we use the GPRS Shield with a FEZ Panda. It works very well if the amount of data to send is < 1000 bytes.
When we sent more every time we get “ERROR” and nothing seem to be sent. That’s why we use handshake Xon/Xoff -> AT+IFC=1,1 and set it at the UART of the CPU but this dosn’t help. We also put the message in some packages < 1000 bytes with a delay but this also didn’t help.

Dis you have an idea or some code that works?