kshady has a point. It could be your MAF starting to take a shit. dirty. loose plug. maybe the seal on the sensor itself is going bad and getting moisture in there.

could always try unplugging the speed sensor too and see if that changes anything. Won't have a speedo, but the ecu won't know how fast it is going so that rules out the speed related idea.

could be something really dumb, like the ECU thinks it is in 5th gear all of the time and you're hitting a rev limiter.
hell, unplug everything off of the tranny and see if it still does it.
It would be pretty dumb if that was the problem, but you never know.