Our 14 spark cvt acted like that. On the highway the tach would bounce at a steady speed. Dealer replaced cvt no questions asked. Then it did it again shortly after so they replaced it again and we sold the car since warranty was almost up. This was with under 60,000miles on it.
My 14' Mirage with 287,xxx kms acts like that sometimes when hot. I just let off the throttle and let it upshift. I don't drive very far with it though, only about 20km each way a day.
If you can get warranty coverage I'd give that a try. Depending on the warranty you have. A dealer would probably claim you gotta follow the severe schedule.
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View my fuel log 2014 Mirage SE wussie cvt edition. 1.2 automatic: 37.7 mpg (US) ... 16.0 km/L ... 6.2 L/100 km ... 45.3 mpg (Imp)