Yes, speed density cars are known to have more of an idle problem than mass air cars. I think the difference is that a speed density car uses only the preprogrammed info inside the computer to tell it what to inject and when, based on general info from the throttle position, manifold vacuum, air temperature and what have you. But the mass air cars are using that info PLUS measuring how much air is actually being sucked into the motor, which gives the computer a much more accurate picture of how much fuel it needs to inject. And that difference allows it to compensate for changes in the engine, such as heads, cams, etc. That's why speed density cars fall down when you do mods on them -- they think that the car is stock, and don't have any way to tell otherwise. But you should still go down the checklist of possible problems contained in the "sticky" about surging idle issues at the top of this tech forum list. It contains a pretty comprehensive list of things that can affect your idle.