Auto Bracketing?

My D90 has recently begun to over/under expose. Especially in burst shooting. I'm in aperture priority 80% of the time: manual the rest. Yes: bracketing is off. Generally, the first shot will be dead-on, with the others going a good stop or two in either direction. Getting annoying.

Were you under cloud cover with changing shadow?
Aperature priority constantly meters.

^^^^ yes my thoughts exactly. Where you aiming at diff areas while shooting? Where you zooming using a non fixed aperture lens while shooting?