Driving uphill on Auto transmission?

The car will shift down to 3rd or 2nd gear or even 1st gear if the hill is that steep, or you're carrying a heavy load. That's normal. It needs to downshift to have enough power to pull the car up the hill. If you think the RPMs are too high, your best option is to slow down. If the car is shifting back and forth from gear to gear, you should downshift out of overdrive, and let it work in 2nd and 3rd gear as it needs to. The reason is that overdrive is designed mainly for flat open roads at a steady speed. Shifting back and forth a lot will cause the transmission to heat up, and could lead to an early failure. I've had that happen, and it's not a cheap repair.