Erratic shifting from 3rd to 4th

[k98shooter]

Son's car. 2012 Yaris hatchback automatic with just over 60k miles. The car ran silky smooth until now... No CEL, no stored error codes.
This happens after engine warms up to operating temperature. When driving at 20-40 mph, at a steady speed, without accelerating, the transmission starts shifting randomly from 3rd to 4th. Appears randomly fluctuating between 3rd and 4th. Often it shifts to 4th prematurely, so there is a sensation of power loss as the RPM is still too low for 4th gear. The car runs just fine at highway speeds and shifts through gears just fine when accelerating hard. He took the car to Toyota dealer who said it appears transmission issue, but could not give more specific answer (even though he paid $150 for "diagnosis").
Your input is highly appreciated. I suspect on of the solenoid valves, just not sure which one. Perhaps best solution would be just to replace entire valve body?