no way to really put this nicely, your lube is junk.
you need to order some lube meant for Spool valve markers.
Violent pb jelly
try one of those. I use lurker lube on my DP guns and they all shoot flawless and the first shot is always in the 280 range. no drop off like you are having.
Yeti PB Balls Out Photography Becker Industries
DPOT: Hotter than a 2 peckered billy goat
MacDev Clone GTi