Shifting back and fourth between 4th and 5th gear.

Good morning all.

I have searched and read info on this site about a few of you that for one reason, or another had this problem. I never found a clear diagnosis, or remedy that was used for those that had the issue.

My issue:
My 08 Passion Coupe with 190k always shifted smoothly, however I needed to replace my from wheel berrings and all motor mounts a month, or so ago. The day I picked up my car from the mentioned repairs it was shifting back and fourth from 5th down to 4th and back up to 5th repeatedly. Car is in 4th for 30 secs, shifts up to 5th for 10 secs then goes back to 4th. This happens with constant pressure on the throttle on level ground. I understand if going up a hill, or increasing my speed, etc. the car will shift to get to the proper gear for the speed at that moment. When in manual mode there are no issues with shifting. Again, before motor mounts and berrings my car shifted fine and all other gears and shifting was normal. This issue only happens with 4th and 5th gear.

Wheels bearings are good to go and the wheels spin freely without drag. I plan(today) to put a little more air in the tires as some have suggested in other threads, although I don't see what difference that would make if the tires are at factory specs. I also don't understand why no other gears are effected. The mechanic said the throttle wire may be out of whack because of the new mounts, and may need to be adjusted. (I think that's what he said, trying to remember) Any ideas on what this issue can be???

Thanks in advance.