What happened?

[uglymike]

Shooting decent groups with my HW35e couple days ago @ my indoor range. From 1 shot to the next my POI dropped 3"  Thought to self, boy you flubbed that one! A couple more shots, same thing, 3" low. Cocks the same, no untoward sounds, but the shot cycle is now........sluggish. Report isn't the same, even to these deaf old ears. Ran it across the crony this PM, sure enough, lost 100fps. This is a new, tuned rifle with less than 2 tins through it. No wild fluctuations in fps, now 670-690 with JSB 7.87gr.

[Roadworthy]

You can check through your cocking slot to see if your spring is intact but my best guess would be either the spring or the piston seal.
Of course you'll went to check simple stuff first so try the tissue test for a possible bad breech seal.