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When you removed the plastic covers to expose the spark plug wires, did you check al lthe wires if they are in good condition? No break in insulators? Lead wires are good?
Misfire...? Ignition problem. No a valve cover gasket (VCG) will not cause a misfire, you've got problems in the ignition system, being plugs, wires, cap, rotor, coils, wiring to ECU could all be possible culprit in a misfire. If your timing belt was off, you'd have different symptoms. I guess if your timing belt tensioner is loosing tension, it would cause the belt to bounce around and change timing depending on where and how the belt was stretching. Check the easy stuff first.