First thing that comes to mind is to suspect the oxygen sensor in the exhaust. Not sure you have one in that car - but thought I'd throw in the idea.
Someone might comment on generally what causes high NOx emissions. I think its fuel mixture that's burning too hot, causing the nitrogen in the air to burn also.
Hmmm. You have the right spark plugs in the car with the right heat range?
I always tell people to make sure they are running premium fuel for a few tanks before the test - you probably already are doing that.
Just grasping at ideas here........