Nitrous cutting out

I have a '90GT with aftermarket heads/cam/intake. I went to my local drag strip on Fri. night. I recently installed a bottle warmer kit in hopes to get the most out of my nitrous system & finally get some consistent performance out of my NOS #5115 150 dry shot. Well the bottle warmer did help out. My car went quicker down the 1,320' for sure. The nitrous worked perfectly right off the launch, through 1st, 2nd & 3rd gear...but when I shifted into 4th it pretty much felt like the nitrous turned off. So I tried one more run on the juice only to find out that the nitrous wasn't even working this time. At first I thought the problem might be that the vacuum line had blown off the nitrous regulator, which would have made sense seeing how that vacuum line is what boosts the fuel pressure up to 85psi. to mix with the incoming nitrous oxide. No such luck. The vacuum line was secured and in place. I continued to look at all the vacuum lines in hopes to find one that had fallen off of the manifold or dried out & was cracking. But...everything looked fine. Then it dawned on me that it has been years since I've rebuilt my nitrous solenoids. Do you think this could be my problem? Oh, & I have plenty of fuel voume & pressure. I have a 155lph intank fuel pump and an MSD 200gph inline fuel pump. So it's not the fuel pressure safety switch shutting off the flow of nitrous due to inadequate fuel pressure.