the width of the memory bus of mx4000 is 32 bit, maybe thet is the problem - try turning on the safe mode in the settings - that will make starfury put all texures in the system memory, and it should be fine.
A tad bit faster in safe mode, and Kwok, the only common element is that the game is _running_. When things are on-screen, it's slower, but a lot of the time, even relatively empty screens gave depressingly low framerates.