Quecetel M95 GPRS connection question

I am using M95 module from Quecetel. I am sending data to my server and facing a weird problem. I wonder if someone else can give me some idea.
I use AT+QISEND=128 to send 128 bytes at one time. Then I use AT+QISACK to see if all 128 bytes have been acknowledged. This process is in a loop.
BUT after like 512 (rarely 640 as well) bytes the connection closes by itself. Does anyone have any idea what setting I might be missing?

AT Command manual is at: http://www.sos.sk/a_info/resource/c/...5_ATC_V1.0.pdf