I wrote about this problem last month. To date, it is still not solved, but I may have come across the problem.

The symptoms are that during the environmental tests, the CO & HC's where practically non-existant. The NOx ranged 20 to 50% over the posted maximum (its been tested 3 times now).

I had replaced the cat, cleaned throttle and replaced some suspected hoses. O2 sensor seemed to be working correctly (.1 to .9 volts ocillation). Started it up after setting the base idle from hot and it would hardly turn over at 500rpm.
I had taken apart the ICV to clean and managed to re-assemble improperly. So off I go to the pick and pull, get a new-used one for $20. Seems to work properly at the pick and pull (applied 12v to centre pin, grounded outside elements one at a time and the valve moves). However, when installed, the engine raced to 1200 rpm. Found if I rap the ICV with the butt of a screw driver, the engine will reduce throttle to where it idles correctly. Stop rapping, and the engine RPM start to climb again, stabilizing at about 1100 to 1200rpm.

Anybody else have this problem? I cleaned the valve with carb cleaner but it almost appears that the valve is sticking. Think I have found the high speed leak if the idle control circuit is never shutting down. Will retest to see.