Latching on to Jm.'s comments, you're facing some issues.
I was pushing 240 pounds when I owned my NRS a few years back, and my AD12 shock was pushing it's red line at around 250 psi. This suspension is tough on shocks, because despite its modest suspension compression ratio, the shock doesn't get the benefit of any sag -- meaning it needs at least 33% more air pressure than a typical single pivot or 4-bar bike.
Coil is a bad idea for two reasons. The suspension needs a rising spring rate, which an air chamber provides as the air is compressed. Any properly sprung coil you install on the NRS is going to be worse than what you're experiencing now. In addition, the spring needs to be *perfectly* tailored to your weight, to hold you just on the cusp of sag. Too much spring, you'll be esentially riding a hardtail until a big enough bump force comes along to displace things, and too little and you're suddenly in bob city.
Make sure your AVA chamber is set all the way to the minimum. Failing that, I believe you can retrofit your AVA chamber with a standard Float chamber, which will be somewhere in the neighborhood of two-thirds your AVA's smallest volume, and will provide you with the rising spring rate you're looking for to solve your trouble.
I quickly realized my NRS was not the bike for me -- great for it's hardtail-like racer qualities, but definitely not a compliant trailbike. Re-evaluate your needs. I ended up switching to a dw-link bike and actually felt more efficient on it. And there are a lot of less severe NRS alternatives out there if you can't get this ride dialed.