My 2008 RX400h does not maintain set speed on cruise control. Speed increases on down hill and does not increase going uphill.

 

When you say speed increase, do you mean that the speed increases beyond the set speed? I would think that if you want to maintain the speed with engine braking, you would use the 'B' mode on the shifter.

The speed should not "increase" beyond the set speed when you are going up a hill, or are you saying that you have trouble forcing the vehicle to accelerate when you push the accelerate button on the cruise control?

 

What happens is on long downhill roads the speed increases instead of being retarded to maintain set speed. Going on long uphill roads the power is not increased to maintain set speed.
Putting gearshift in B cancels cruise.

 

It works correctly on the downhill (for RX 400h) since engine and brakes are not involved in D (use B for engine braking), but something wrong with driving uphill if your speed was > 30 mph (at <=25 mph cruise will be deactivated) - RX 400h has enough HP to keep the speed

 

My 2006 has a similar issue. It keeps speed perfectly in level ground and uphill, but goes loses speed going down hill. On hilly roads, it is constantly changing speed.