LeapFrog

Just came across a situation that I'm curious about -

Yesterday I was on a long bridge with a slight (maybe 3 degree) downgrade, traveling at 70 mph with the radar cruise enabled. There were no vehicles in front of me, and no visible obstructions ahead. Vehicle mode was set to Normal, and the transmission was in Drive.

The 3IS topped the bridge in 8th gear at 1800 RPM, and shortly after starting down the bridge slope the transmission shifted into a lower gear (either 7th or 6th, I didn't think to check) and the engine RPM increased to 2000 RPM. About five seconds later the transmission upshifted to 8th, then back down to the lower gear a few seconds after that. It continued this hunting pattern most of the way down the slope.

I suppose it is possible that the cruise control was trying to maintain the 70 mph speed setting by downshifting the engine, although I think simply reducing the throttle setting would have been sufficient to slow the vehicle on this slight downgrade.

Anyone else have this experience? It could be a programming problem, or it could be "normal operation".