Nitrous problems HELP!!!!

[Blaster]

I recently purchased a nitrous works 125hp plate for 91 GT, it has a 331 in it with ported performer heads, RPM intake, bigger mass air, inj.,4.10's, MSD digital 6 plus, window switch, etc. Anyhow, i got it set up and the purge leaks a tiny bit so I guess no big deal until i can get a new one. My problem is when I get on the car with the nitrous turned on it pops mainly in 3rd and 4th gear at the top ends of each gear. I played with the fuel pressure and leaned it out about as much as I thought was safe (30 psi KOER) but it still pops abit. I have a custom burnt chip too which I'm sure was tuned for my original pressure and probably pretty lean already. I put NGK R BKR5EY plugs in the car becuase I was told they are 1 stage colder than my old ones (NGK fr5 I believe). Could it be the plugs?? My old best was 12.80 at 108mph N/A, and I ran a 12.33 at 113.5mph with the nitrous w/a 1.80 60"ft time. Any ideas??? I was expecting to drop at least 1 second with a 125 wet shot!!!