maybe its just the simple fact that the kt4 board blows when it comes to some games
i know that when i had my msi kt4 board and changed to my current nforce2 board i was extremely amazed on how much better it became with just a board change.
i had slowdowns on the exact same games as you do.
mohaa was a real pain in the ass to play on my kt4 board but not on the nforce2 board ;)

one other thing you havent answered is this, do you have the latest via drivers? the hyperion drivers installed?

here is the link for the hyperion drivers.
http://www.viaarena.com/?PageID=300