i have a 2004 g35x. i've install a k&n drop in filter. when i brake my i see that my needle bounces up and down from 500rpm to 1000rpm. it only does this when i slow down to brake.

mine does it as well, but I sort of figured maybe it's from the transmission downshifting?

yeah it seems weird to me because i never seen it on other automatic cars ive driven before.

Its brake assist...the car uses the engines rpm's to help so the car down taking some pressure off of the brakes system..my 04 sedan doesn't have it but my 05 V6 altima does...

I have the same problem on my 04 6MT. It only does it when the A/C is on...I don't know what causes it. Maybe try to reprogram your ECU?