Windows Media Player AVI video stutters

When I try to play my avi files on this new Windows 7 Ultimate 64 bit OS, it stutters every few seconds.
It starts off going fine for about 5 seconds, then starts stuttering.
I don't get this problem with any other things like games.
I never had this problem either on Vista with the same files.

I've tried to use Win7DSFilterTweaker to swap the codecs that WMP uses to ffdshow instead of the Microsoft one, as I've heard that it wasn't as compatible.
I've also installed K-Lite Codec Pack 64 bit, but to no avail.

Currently I have a:
Quad Core Q9550 @2.83 GHz
ATI Radeon HD4870 (Drivers up to date)

Thank you for your help!

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