Af I drive about 15 miles and stop for what ever reason after a few seconds my truck will idle down to about 450 RPM. I recently changed spark plugs, changed the oil, and cleaned the air filter( K&N). If I bump it up to neutral or park it will go back to a normal idle speed. The only mechanical problem with her is I think is a cracked drivers side exhaust manifold. Any suggestions for a resolution to my problem would be greatly appreciated. Plugs are the NGK factory replacements.