Our team had this exact problem and after a bit of troubleshooting it came down to the brand/model of the USB-RS232 (Serial) convertor you use and the way it translate/converts the data in either the hardware (chipset) or software.
Our team has great success with the following adapter(s):
Keyspan Serial to USB Adapter
http://www.compusa.com/products/prod...to_USB_Adapter
Port_Authority_USB_Serial_DB9_Adapter
http://www.compusa.com/products/prod..._DB9_Adap ter
Also changing some settings in the adapters has helped us improve the speeds
to near native Serial speeds. This includes
1. Making sure in the Windows device manager your virtual serial port is SET to the default settings WITH 115,200 baud rate
2. Make sure that FIFO buffers are enabled in the adapter in the driver settings. This is a bit complex but it boils down to the way the adapter negotiates with the RC.