Hillbilly's on the right track. I did a poor power/die-out diagnosis on an '89 Cougar 3.8L, turned out to be bad intake sealing. Sussed that out with a can of carb cleaner, confirmed it with a smoke test. Pull and cover the PCV, then shoot the cleaner into the PCV grommet at idle. If there's a leak, just like under a carb or the outside of an intake, the engine speed and note will change.