hmm im not sure about the ethernet card, mine actually has two ethernet ports. Im not sure if the forcedeth causes this problem I just used the Marvell because it has Linux support for gigabit ethernet while the nForce one did not(only 100meg). If dmesg is reporting timer issues thats your problem, as soon as I got those timer issues to go away everything was fine. Im using the gentoo-sources attached is my kernel config incase you want to look at it or try it(not sure of your distro or what sources you use but you should be able to get it to work with little effort). To see how far off my clock was getting id start compiling something and then Id do this
date && time ./setclock && date
setclock is a little script I wrote
rdate -s time-b.nist.gov
hwclock --systohc --localtime
date tells you the current time, time tells you how long a program/script took to run, and setclock gets the current time from nist and sets the system time to that.
Finally about my motherboard im using the latest stable BIOS(1009) nForce ethernet is off, Sil SATA is off, and the on board sound card is off(I have an Audigy).
AMD64 X2 4400, ASUS A8N-SLI Preimium, eVGA nvidia 7800 GT, OCZ Platinum EL-PC3200 2-3-2-5 2gig 200gig WD SATA150, SB Audigy 2 Platinum, Pioneer 108, Dell 2405W, all running on Gentoo Linux ~amd64.