Same problem here and no idea too.
I tried to use a Intel Pro/100 CT network card instead of the Realtek onboard network adapter, but the problem is still the same.
So if you are thinking of replacing hardware, make sure you can return it in case it does not help.
Are you using W7 x64?
As far as I know my onboard NIC is by Intel, not Realtek. The PCI card I added is a Realtek, and it works better.
I tested another PC with W7 x64, that has an nVidia NIC - no problem there.
I opened a customer support ticket with Intel support. I hope they get back to me and solve the problem.
Yes, windows 7 professional, 64 bit.
I have an ASUS motherboard with Realtek NIC and I deactivated it to test the Intel card.
First I thought it would help, but that was wishful thinking. The problem just needed some hours to occur.
Still waiting for Intel support to solve sthi. In the mean time I found a way to restore normal DPC latency without rebooting: Go to Services and restart Base Filtering Engine. After this you need to restart Windows firewall from the Action Center in the systray .