If it is a TL (he of FB fame) 22mm skirtless piston I have the same in my full length TX .177, and have found QYS to be the best
I tried all the different headsizes of JSB Exact, AA Diabolo, FTT, RWS, in the rifle as standard, when it had a TbT short stroke kit, and in its current state, and nothing even came close to the consistent FPS, grouping, and accuracy of the QYS Streamlined 8.48gr 4.50mm head
Mine is running around 10.5ft/lb, standard AA spring, no preload washers.... it did take three strip and rebuilds to get the stroke length correct but it is now almost recoilless, completely impervious to hold (although I still wouldn't clamp it in a rest like a PCP!), and single figure FPS variance
Give TL a call when you have enough time to absorb everything he will tell you.. the man likes to talk! My best guess with inconsistency like you describe would be breach seals, excessive amounts of grease/lube or a spring that it is out of shape, poorly dressed, or has been heated and squashed
TX200 .177 21mm TL | TX200HC .22 22mm TL | Prosport .22 22mm TL
HW40 | HW45 | Diana Mod5 | Webley Senior....... Because PCP is like kissing your sister (apparently)