Stuttering a lot with Mario Kart Wii

Hi Guys,

Having some problems with Mario Kart Wii.

The game particularly stutters when in the menus, and at the beginning of races.

When playing with 3 players the whole game becomes very stuttery.

I've used a fresh install several times and some newer builds, same problem.

PC:
i7 870 @ 3.3Ghz, AMD R9 270x, 4GB Ram, Windows 8.1

Any help appreciated, cheers.

EDIT

My friend also has the same problem with an i7 3770, AMD 7970, 8GB Ram, Windows 8.1