Tip that I learned the hard way. If you're running this setup, as far as I know it doesn't apply to 08 guests, and you notice erratic, high, and even negative PING responses (talk about predicitive networks!) you need to edit your boot.ini file and add the switch /usepmtimer
Article that helped me when this happned on my recent NPM install. Interestingly no alerts were triggered even though my PING times had risen to as high as 1600ms.
Hopefully this saves someone else some time and head scratching. Our server wasn't ready for full production yet, but it was getting close and then this anomaly cropped up. Does "wonders" for your 95th percentile and other averages though. :)