Have you tried putting windows into 16 bit colour mode (that used to work for me when I had similar "delayed chug" problems).
Also, if you are using a HUD then try turning it off when the chugs set in. If that fixes the problem then you might want to either try using a different HUD or else altering the settings so the HUD displays using a different method (the transparent "layered windows" method can be real slow on some graphics cards...).
Juk