Developed rough Idle several weeks ago. Replaced defective cat with no improvement. Noticed a hiss coming from what I beleaved was the flame trap. Replacing flame trap and hoses did not improve idle. Tried to isolate the leak listening with a fuel line hose. The leak seems to be in front of the flame trap between 3&4 cylinders intake. Spraying carb. cleaner in this area smooths out the idle but makes the greatest difference between the intakes, not around the observable portion of the intake gaskets. It also races slightly when started. Am I missing something here. The intake gasket seems to be the most likely culprit. If so what are the issues involved in replacing it and how much would a mechanic be expected to charge.