How is your cooling system? These things are prone to trouble in that area (fan clutch?), and that can cause NOx levels to get up there. NOx only becomes a problem once the combustion temp. gets above 2500 degrees f. which can be caused by overheating. You may also try getting rid of some carbon buildup in your combuistion chambers which can cause "hot spots".

'91 LWB, 31x10.5s, KYBs, A/T "I look like Robert De Niro, I drive a Mitsubishi Pajero" - Billy Bragg (not the original lyrics, but that's how he sang it when I saw him in Dublin in '04)