Hello,

I hope I could offer additional help here. Please check the large vacuum hose from the brake booster to the manifold. With heat and time this hose could harden causing leaks at the connections. Maybe its time for replacement.

Also please check the hose connecting the IAC to the intake manifold. This hose is hidden in-between the intake manifold and usually overlooked during maintenance. Tighten the connections.

Another remote possibility is the exhaust pipe gasket is leaking letting additional air into the system. Its a two-hole gasket secured by three bolts at the end of exhaust manifold before connecting to the front exhaust pipes. This is the most 'stressful' part of the exhaust system as most vibrations are transmitted here.

Please go to the address below for a photo of this part (labelled exhaust manifold to front pipe gasket)
http://www.vlvworld.com/indexframe.html?900/Section_2_11.htm

In my car I have been replacing this gasket about once in every two years until one exhaust repairman suggested to install a short flexible pipe just before the catalytic converter. In the original design this 'flex pipe' was not included. This is to isolate the exhaust vibrations from the front manifold end making it less stressful. So far its been almost 5 years since I've replaced the gasket.

Please go to the address below for a photo of this part.
http://www.p-wholesale.com/cn-pro/9/418to1/exhaust-flexible-pipe-2-394391.html

I hope this helps.

Amarin.