i have a 2000 f-350. when I got the truck. Ad said newer trans and heavy torque. every morning I come down a pretty step grade. so. this morning I did something different. came down in drive with the overdrive off, I didn't hit my breaks at all. at 50-55mph my trans shifted, rpms went up, and 5-10 seconds later did it again. but the rpms just went up a little more. don't think its a bad thing. just asking what is causing this. thank you!!!!