Being it's that good, yet an issue...
Timing chain, silent stock type could be contributing to the issue..as could burnt or sunk partially sealing exh valve seats.
Can happen with that low mileage and the factory did a marginal job on the induction hardening.
Seems like it would be a good candidate for a re-ring job . With a fresh hone...new rings, new chain n gear... it would be like new, for real.
