My 1991 190E 2.6 Won't Pass Smog

I recently purchased a 1991 190E 2.6 from a friend. The car has 220,000 miles. The car looks and runs great, but it has failed numerous smog tests (6). I have put over 2000.00 into the vehicle to try to get it to pass smog. I have replaced the Catalytic converter, EGR valve, plugs, distributer cap and various vaccume valves. I have had the car Pre-Tested at a smog station and it passes, but when I take the car to a Smog Test Only Station it fails. They say there is too much NOX comming out of the tailpipe. Any ideas why it continues to fail the smog tests after all of these repairs?

Thanks for your time.

Zack