I'm having a strange problem with my .30 cal FX Impact. The adjustable air regulator is set to 150 bar, the power hammer spring adjustment is set to max, and the valve return spring adjuster is opened. With these settings I expected to reach 840-850 fps with a 50 grain pellet, but when I study the chrony the shotstring starts at around 650 fps. After 10 shots the velocity goes up to 730-740. After about 20 shots the velocity rises to 770 fps, and finally, after about 30 shots, the shotstring reaches a maximum velocity of about 810. I can't feel or hear any obvious leaking, although the rifle losses pressure at a slow rate (maybe 20 bar a week). What could be wrong? I would be very grateful if someone could point me in the right direction.