the rocker cover gaskets are not relevant to the fault codes. Sounds like you might have a pcv/ccv leak somewhere.
if the valve timing is off, it could register as a random misfire because the off valve timing can cause a compression leak and that combustion cycle will be off.
if you want to have a play, pull out one of the VVT solenoids and check for sludge and debris. You can access the right side one easily but the left side you cant remove without a lot of other work.
like Southark mentioned, you could have blocked VVT filters. There is a thimble gauze filter on each bank, behind the timing cover gasket.
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