Another 1991 Fails CA emissions... Help!

[Shinto]

I've read all the posts about failing emissions, but haven't come across my particular problem (close, but not quite).

I've had the car since it was about 4 years old / 4,000 miles and have since added Borla exhaust and an X-pipe. The car is otherwise unmodified and completely stock. It has always passed smog with flying colors in the past and the last test was only about 8-10k miles ago.

Since the last test: I added the X-pipe and changed the alternator (by removing the throttle body, not the plenum). I just changed the oil to Mobil 1 High mileage about 400 miles ago to protect the cams. The car has 39,800 miles.

I had to go to a test only station since the state has selected me as a "high emitter profile" and here in Sacramento we have to do emissions testing under load on a dyno at 15 and 25 mph.

I filled the tank with 91 octane (I always run super) and drove it on the freeway prior to the test to warm it up and blow out the carbon. The coolant temp was around 220 and the car was audibly pinging during the test.

The failure is in NOx emissions at 15 mph (and nearly failed at 25 mph). All other measurements were passing and quite low.

15 mph / 1298 rpm
CO2: 14.7 (no limit)
O2: 0.2 (no limit)
HC: 14 (limit 112)
CO: 0.01 (limit 0.72)
NO: 1135 (limit 778) ***

25 mph / 2106 rpm
CO2: 14.7 (no limit)
O2: 0.2 (no limit)
HC: 13 (limit 86)
CO: 0.01 (limit 0.60)
NO: 527 (limit 717)

What has failed? The tech suggested maybe carbon deposits (but I blow it out quite frequently and did just prior to the test) and the coolant temperature being high.

Man I hate California! I already can't drive my '78 Trans Am with 48k due to failing as a gross polluter an now this @#$%#$%. Wish I could just bribe someone...