I agree with Slayer's evaluation of the cause (although I don't know if I'd say MTM2 wasn't "designed" for high frame rates, as that makes it seem intentional, and I believe it's more of an accidental bug or oversight by TRI that they didn't recognise on the hardware at the time.)
Something in the time delta between framerates seems to affect the decisions made by the AI trucks. As the framerate increases and the time between frames decreases, the steering inputs of the AI trucks also diminish, to the point that they run wide everywhere on corners, drive slowly and miss CPs.
I saw this most clearly when I had to turn Vertical Sync off in my global graphics driver settings on my Pentium4 computer with Geforce 6800 graphics card. The Vertical Sync (or Vsync) synchronises the framerate of a game with the refresh rate of the monitor, which is typically about 75 frames per second. With it turned off, on such powerful hardware, the game was running at about 150fps. The AI trucks pretty much didn't steer at all, so they just coast off at every corner. With it on and locking the game to 75fps, they do a bit better, but as Slayer notes they do better still if the framerate is even lower (they can steer more aggressively).
For cool260z - it's hard to say, but something must have changed on your computer to make the game run a lot faster. (Tip: while playing, type in F-R-A-M-E to activate an in-game framerate display.) Unless there's been a hardware change you've not told us of, then the obvious one I think of is the Vsync, which you can turn on or off in the control panel software of your NVIDIA or ATi graphics card (MTM2 doesn't modify it, but you may have?).
(Sorry for the novel, but this behaviour of the game does fascinate me somewhat
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