If you want to see for yourself if it is a burnt valve or head gasket, one quick way is to do a compression test on the engine. If the results are within specs., then you might look at other areas.
Another thing to do would be to hook a vacuum gauge up to the engine and then drive around. If it is stock you should get a constant reading of about 17-21 in. Hg. This gauge will also indicate bad valves by dropping significantly rhythmically at idle.
Good Luck
Adam
