My '02 Q45 does whatever the hell it wants when I step on the gas. The downshifts are almost always incredibly late, and even in "manual mode", the downshifts are often > 2 seconds after I move the shifter. But then every once in a while my Q roars off the line like it's supposed to, so I know it's capable. I did get the ECU update or whatever it was from the Infiniti dealer, there's been no noticeable difference. Has anyone else experienced this? I'm worried it may be a sign of my transmission going, but my '02 Q only has 83k miles. What can I try? Can I get an aftermarket ECU for this generation?? Thanks.

Have you checked ATF fluid level and condition? Scan for codes.
In what type of conditions does it drive well?