High NOx can be caused by an over-lean mixture, as in vacuum leak. Investigate that cruise servo leak - you may have to cap the vacuum line temporarily to see if that fixes it. Also, screwed up timing can cause high NOx. Have you checked the OBD read-out for any fault codes like faulty knock sensor? I believe timing advanced to the point of pinging can cause high NOx. If your car has an EGR system that's not functioning properly, that can cause it. Basically I think it's any situation that can cause unusually high combustion temperatures.

I used to have a Dodge Dakota that was always right at the limit (or slightly over like yours) for NOx level. The guy that I went to for the emissions test would turn the heater on full blast in order to try and keep the engine head temperature down. I wonder if a cooler thermostat would help in some cases.