Are you going into boost when this happens? Had a similar situation with a friend'* 00 SSEi, same RPMs, but during freeway driving. If it didn't come out of lockup, or down shift, it would stutter like that, RPMs changing up and down maybe about 200 RPM. It turned out to be a voltage/current issue.
I removed and cleaned all battery connections, under the hood, and under the seat. I didn't find much of a problem, until I got to where the grounding strap is bolted to the floor pan. It'* attached to a painted surface, and the threads on the bolt and in the floor were pretty rusty. I scraped some paint off from the floor pan, cleaned up the threads, applied dielectric grease, and included a star washer to help bite into the metal a bit better. That solved the problem, in that case.
Black 2000 SSEi, Silverstar Headlights, Angel Eyes fogs, 3rd brake light overlay, hi-flo cat, drilled 180 degree thermostat, Intense FWI, PCM, shift kit, and 3.4 Pulley. Solid front mount.