To check for blow-by do a leak-down test. The thing that sticks out in my mind is fouled plugs and uses oil. It almost sound like a "little" oil is getting in the cylinder. I'd check the PVC valve to see if it popped off or see if the PVC valve is pulling to much oil in the intake. Sometimes increasing compression and more ring seal will result in excessive crankcase pressure. To a greater extent I'd also check the valve seals.
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