Slipping out of Overdrive??

You'd have to drive the car to see exactly what it's doing. I see a lot of people get on and off the gas constantly. They'll press on the gas and get going and then let off and let it coast for 100 feet. They do this all of the way down the road and that will cause the OD to switch on and off.

You should drive the car on relatively flat roads at a steady speed and see what it does.

Before I put 3.55s in my F-150 it would go in and out of overdrive on the slightest hills sometimes. It did that for 100,000 miles so I'm pretty sure it's supposed to do it.