do a compression test..if two adjacent cylinders have low compression then you prolly need a headgasket(and check the head for straightness and cracks between the sparkplug holes and the exhaust valve)....if only one cyl. has low then there could be a camlobe worn or something in that cylinders' valvetrain..if the test shows all 4 fairly equal then you need to go to the tune up basics: check the plugs,cap,rotor,wires,vacume leaks