Subaru "Check Engine" P0026 issue won't go away!

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Have a Subaru Forester 2010 (non turbo) model with ~110,000km. For the last 20,000km I have taken it to Subaru mechanics about 5 times with the same issue - Check Engine light comes on after longer highway drives (longer than half an hour) at high speeds - not during these drives but usually the next time the car is driven. Every time the computer shows a P0026 code (right OSV valve). They claim to have now replaced both the right and left valves but the issue came back last week, right after a fresh oil change. Please help!