have some trouble with my disco

This problem has happen to both of my discos now. The last one I bought was tuned by Mac1, in which I am very pleased with his work.
I had not shot the rifle much from new until the other day when I started to test out different pellets for accuracy at different power settings with the Mac 1 tuned disco. I had shot about 150-200 shots with different pellets at different power settings when all of a sudden the power from one shot to the next dropped quite a bit and the POI dropped well, about 2" at 25 yards, now the disco is seams to be shooting at half power.

This problem has also happen with my first Disco in which I still have but the other disco only does it one out of every 5 shots, so I know that it is not something that Tim may have done while tuning the rifle.

Since it has happened to both rifles it has led me to believe that it maybe a common problem with the discos. Have any of you disco owners had this problem and if so how did you fix it.

I have checked the probe O ring and both rifle seam to be fine.

I would like some input and trouble shooting from you air gun experts out there.

thanks

Luis
Gilbert AZ