There is something called Buffer filling that might be the problem here. There are a few articles about it in my journal. Also read about it on a few other forums.
Control Panel->System->Hardware->Device Manager
‘Right click on each communications port listed, and select Properties. Click the Port Settings tab. The Bits per Second setting should be at least 115,200 and the Flow control should be set to Hardware’
Also, he goes on to say
‘Next, click the Advanced button and select Use FIFO buffers and move the Receive and Transmit Buffer sliders as high as they can go. You can lower them again if you need to but I wouldn’t expect that to be necessary.
Make sure the NICs are set to 100Mbps full duplex rather than Auto.’
Hope it works for you mate.
Cheers.