SerialPort Data Loss in C

Hi,
I've connect my hardware serial output to the COM1 port of my Mini2440
device. the data are transferred at 115200 baud rate. Each packet of data
is 6-bytes. When I use 300Hz(300*6 Byte/Sec) sampling the received samples
are correct, but when I change the sampling rate to 600Hz, some bytes will
be lost. It seems that the buffer of serial port is very low. I can not
change the serial port buffer size by SetupComm() method in C++.
When I use a USB to Sarial Interface to feed data through USB port of
Mini2440 the problem will be solved and no data will be lost.
Also, In C# program I see that the serial port reading has Memory
Leakage!!!
Would you please help me to solve these problems?

3600 words/sec * 11 bits per word = 3900bps. The RX FIFO on the 2440 and
6410 has a depth of 64 bytes. The interrupt handler should be able to cope
with that very easily. Are you sure the bug isn´t in your code? Are you
polling the com port or are you handling events?

Sorry 3600 * 11 = 39600