Windows 8.1 - game lags and is not playable

[BearEnergy]

When I try to run MX Sim it is super lagy, to the point where the game is not playable. Worked perfectly with windows 7 trial, then stopped working when I upgraded to 8.1. I installed from the link given in the email after purchasing the game. When I open the graphics/sound setup in game the driver info shows gdi generic 1.1.0. I am pretty sure all of my drivers are up to date, and my best guess is that the game is trying to run off the on-board graphics. I've attempted disabling the on-board graphics from the bios with no luck. I can't play at all because there is just too much lag. If anyone has any solutions or advice, it would be super appreciated! I just wanna race some dirt bikes!

Some System Info (let me know if more info is need):
Windows 8.1 64-bit
mobo: Biostar TA790GX 128m
video: ATI Radeon HD 3870

[jlv]

GDI is the Microsoft software OpenGL driver. You need to install the graphics driver from AMD's site.

[BearEnergy]

I installed the latest graphics driver and still the same problem. To elaborate, it lags on the menus as well. I'll hover over a menu option and it take a few seconds for that to actually become highlighted and clickable.

[jlv]

Does it still say "GDI" in the driver info?

[BearEnergy]

Yes, it says GDI Generic 1.1.0

[jlv]

You still have the Microsoft drivers in that case. You need to go to AMD's web site and install the Catalyst graphics driver.

The 64 bit Windows 7/8 driver is here.