2007 Sonata mileage problem

I have an intermittent problem that is reducing our gas mileage to about 13MPG. When we first purchased the car, the mileage was 17-18 in town and 27-30 on the highway.

I live in San Francisco (the city of 100 hills) one block from the beach. I travel on a street at least five times a week that has a 2% grade. My destination is 315 feet above sea level and about 1.75 miles East of my home.

On the return trip (coming down hill), I often notice that the car is slowing somewhat. This problem does not always happen, but is fairly regular. When I travel down this street with a 35 mile speed limit and the problem is occurring, the car will increase to at least 45mph. When the problem does not occur the speed stays under 35mph. The transmission indicator shows ‘D’ but it feels like the transmission has downshifted. On two occasions while parking, the transmission shifted very hard and the car jerked.

I took the car to the dealer last year and they could not find any problem. Perhaps they do not have any hills nearby to test the car on. I called the dealer Service Manager recently and asked if he had heard of such a problem. While not wanting to commit himself, he thought he has heard of it.

This problem does occur when on the highway. I regularly drive on Interstate 5 North and get 27-32mpg.

Has anyone hear of this problem? If so, what should I ask the dealer to look for?

2007 Sonata SE/Limited

3.3l V6 engine

current mileage 53,000

oil changed 7/21/2010 5W20

all fluids full as of 7/21/2010

all fluids change by dealer at 30,000 miles

car is still under full warranty

Correction: When the problem occurs, the speed is limited to 35. When it does not occur, the speed increases to 45.