It only happens going down this steep hill, but the transmission will do this thing where the engine braking will come on and off and the revs drop and go back up almost like it's smoothly alternating in and out of neutral. It seems like it's going on in 1st or 2nd gear (not sure). It goes away once the car upshifts. Any idea what this might be? The car is ready for a full fluid change (A/T fluid, diff fluid, etc.) so I've mainly assumed it's related to that, but figured I'd check with the experts here before doing a very expensive 10 year type of service on everything if I have another issue creeping up. Happy to clarify or answer any questions beyond what I've described.