Evening all

Just did my first Chrono on my S400 Carbine and was pretty shocked with the results of the shot count! I filled to 180 bar from the reading on the fill bottle (cannot trust the AA gauge). On a 45 shot count the avg FPS was 766 and the PDS was 10.9. However, after just 33 shots the FPS dropped like a stone going from 766 FPS at 33 shots to 740 FPS at 45 shots. Is this normal? I shoot 30 target HFT and this wouldn't be enough to see me through a course, when you I have experienced up to 7 stoppages on a shoot recently! If I up the fill to 190 bar, would this make FPS more unstable, will I get a better sweet spot? I was surprisingly disappointed that 180 bar fill wouldn't even see me through an HFT shoot.
Any advice greatly appreciated!