Vaccum leak in the boot under the air sensor ? General leaks in rubber hoses parts ? Spray carburettor cleaner to see if there is a change in RPM when sprayed at various intake areas such as injector seals, idle valve hoses, etc.
Check vaccum at idle also, should be between 16-22 inch Hg, if not then the problem is on the exhaust side, open the exhauts manifold and see if the problem goes away, if it goes away, need a new CAT.

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Saumil S. Patel