Half of the problem is the gun itself, krakens are always a hit and miss and this has been said over and over again on these forums. Cheap guns are never 'good enough' unless you know what you're doing.
Aside from that, It could be a number of things from a weakened spring, which i doubt, since it takes a lot to weaken it to -100 fps. Or loss of air compression, which seems more likely with the change of sound and velocity loss.
Toss the thing over to a gun doc.