I'm fighting with exact the same issue on my workstation, a Fujitsu W510. Had it already with Win7, then no issues with Win8 and now again the same pain with Win8.1.
My current workaround is using a very old driver which can be downloaded from HP. it is version 11.12.36.0.
See link here: DPC latency + audio/mouse skip and stutter on t420... - Lenovo Community
I did not find the Win8 version mentioned above as signed version so the mentioned driver is the only working I have at the moment. Would really be great to find a solution for this issue.
I'm working on a 1GBit Network.
Best regards,
jokey25