Well, if the cap is still holding 18 pounds, that means the coolant isn't dissapating any heat in the radiator. I still believe it's because it doesn't stay there long enough. If you still have the fan shroud, you might want to eliminate the fan clutch. You shouldn't have to, but doing so would help cool it. How long after starting, during regular driving, does it get hot?
Take care
~Chris
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RACECAR spelled backwards is RACECAR
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