Is this an inconsistent problem? I.e., does it come and go? If so, it's more likely an ignition or fuel delivery problem (e.g, ECU problem, leaky injector(s), poor electrical connection, etc.) than a mechanical problem. I wouldn't entirely eliminate a head gasket or burnt valve as a possibility, but these are more likely to be consistent and exhibit other symptoms at the same time such as a pressurised cooling system, mixing coolant and oil, or chuffing or ticking noise if a burnt valve.