Was the cam belt and other work done before you got it?
Could also be possibly the belt is out a tooth, it may or may not run right with a tooth out depending on how its out, but may not be making power like it should and so you are gassing it more than it should be. If you hadnt driven it before or haven't driven another IS then you may not have noticed.
I would pull the battery for a minute or so to reset the ECU and then give it a try. It might set everything right after cleaning the MAF and other work, or it might throw a CEL if something is wrong and it finally tell you whats going on. I would also get the CEL's read properly, dont wait for the light as it often only lights up on critical items, but might overlook something else.
2001 Altezza Gita JCE10