ricochetv1, it helps to indicate the mileage on your vehicle when asking for members to diagnose problems. If the thermostat hasn't been replaced recently, or never, recommend installing an OEM thermostat for starters. I experienced a sudden overheating problem, similar to yours, with a 104 engine and a new thermostat resolved it immediately.

I suspect your mechanic advised replacing the valve cover gasket to stop a slight oil leak, typically at the back of the engine.

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Fred Hoelzle