This is symptomatic with impending valve seal failure; if the piston head is worn /damaged then air will just bleed out behind the piston. When the valve seal is starting to split up, you get all sorts of erratic behaviour, very common ids the effect of the cocking lever kicking back on opening as all the previous air is not expelled. I would look at the valve area first again, check the little brass cone is clean and not scratched, the spring is not failed, and don't put any grease in there on assembly. Once the valve is sorted you can compress, shoot / check power; then re cock and leave for an hour, repeat, then repeat over 24 hrs; this should confirm piston head condition. Any more info needed, PM me by all means.