Are you maxing out your half-open tcp connections? I don't know what OS you have but W7 allows 20.... You can remove the limit by editing the registry:
Locate and then double-click the following registry key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\EnableConnectionRateLimiting
In the Value data box, type 0, and then click OK.
Exit Registry Editor, restart the computer.
Are you getting dropped packets?