Penn,
It sounds like an air pressure/airseal issue. However, yours is quite severe. Therefore, I tend to agree with Styrak and believe that you may have forgotten to re-attach the tappet plate spring. In this case, the ASN (airseal nozzle) wouldn't be able to establish a good seal with the bucking and you get FPS readings of <100. Don't ask me how I know that...
BTW, this little faux-pas happens more often than you think...
Jawz