Searched but didn't come up with anything.

I have a 99 auto ( i know). Occasionally in stop and go traffic, when I let off the gas as soon as the rpm goes down to around 1500, it then quickly drops to 1000 and bounces right back up to 1500.

The effect is like downshifting/engine braking in 1st gear for a half a second, the whole car jerks to slow down and then picks up speed and continues to coast without me doing anything --- anyone else heard of this happening?