Failed California Emission Tests

1990 Cadillac Deville, 89,182 miles. On very recent, December 10, 2006 initial CA smog or emissions inspection/test at 15 mph, NO reading was 1080 compared to 762 max allowed, or 50% above the legal limit; and at 25 mph, NO reading was 1600 compared to 701 max allowed, or 100% beyond the legal limit.
On the last smog inspection about 2 years ago, the car was classified as a "High Emitter", and it barely passed emissions tested for NO limits. The readings for HC and MO have been way below legal limits for the most recent inspection and the inspection two years ago, SO THE PROBLEM SEEMS TO BE RELATED TO THE NO EMISSIONS.
What car parts could be bad or malfunctioning that are causing the NO readings to be so high? Could the solution be a new cat converter, EGR value, Air Pump, or what?

Thank you very much.