My .177 sub 12 mvt was doing exactly the same thing a few weeks back it was shooting 750fps when it was set to 780 but at the same time I tried a few pellets from a friends tin of pellets which were aa fields and it was perfect so I put it down to a bad batch of jsb pellets I was useing. It's also done it before and that was allways the probe seal even though to the naked eye it looked perfect.
Air Arms tx200 .177 mtc mamba lite 4-16x44
Daystate platinum .177 Mtc viper 10x44
Daystate airwolf MVT .177..Mtc mamba lite 3-12x44...meon valley airgun club.