I also read an article about problems of the same type encountered on 965P motherboards with the Marvell Yukon onboard LAN adapter, which is the same on the more recent P5K Premium.

Users that had that problem did update the firmware of the Yukon adapter to fix the problem, or simply activated the adapter under windows (looks like it was causing problems mainly when disabled). I tried activating it, since I had it disabled since day one: doesn't change anything.

Now, the messages about this problem related to Yukon's adapter were from months ago, so I guess this problem has been fixed by now, and I can't find any firmware upgrade to this adapter anymore...