My RPM meter has been jumping going downhill on crusie control about 500 RPM

Could there be an issue with my transmission? I have a 2007 Ford Fusion and I travel with it over 120 miles a day. I use my cruise control every day. When going down hill my RPM meter usually was at 2000 RPM's and going up hill its goes up to about 3000 RPM's. Lately when going down hill my RPM meter goes down to around 1500 and then jumps back up to 2000.

It doesn't do this with every hill, it depends on how steep. I just started to notice this. This started after I had an instance where I went to turn off the cruise control and my check engine light came on and my car started to shift funny. I pulled over, turned the car off and back on and it was fine. Other than the shifty RPM meter going downhill everything has been ok. What should I look into? The car has over 104,000 miles on it so I do know it needs a good tune up. Thanks, Barry