When I'm driving down a hill, my engine revs and it feels like it's over working on 2007 Nissan Versa

This just started recently, but sometimes I feel like I'm losing power in the car. It mostly happens when I'm going down hills and it feels like the car is trying to change gears but it can't and it gets stuck on too high a gear. It happens on occasion when I'm driving on a flat road as well. As though I lose power for a second, then it comes back. Any ideas what the issue is?

Asked by for the 2007 Nissan Versa