In my experience, the valves on accumulators tend to leak a little. It's a very small amount, but over a year or two you can notice a difference in the ride quality. Since you said it rode great before, my opinion is you're doing more harm than good by letting nitrogen out. There is a little bladder in there and if the pressure in that bladder is too low you can pinch it over a large bump or pothole. My suggestion, get someone with a nitrogen tank and a regulator and hook it up to yor accumulators. Start a little high and slowly let our pressure until you're happy. Back when I had my car I would re-pressurize my accumulators about once a year.
-Aaron
www.artofnoize.com