Not had the exact problem you guys have had (pro psort with vmach spring and guides, standard stock), but will share with you what happened to me a few shoots back and perhaps put another thing into the equation.
Was shooting pretty well about 18 targets into the course, dropping about 14 or 15 to that point. Then started missing badlly. Easy shots were being missed (25yd full kills etc) only took down 3 of the last 12.
Immediatlly went to the zero range after the comp and the gun was shooting predominatlly an inch low and a bit to the left with some shots spot on.
When I returned home, straight to the tools and stripped down. Found that the bolt holding the cocking lever to the compression cylinder was loose. Conclusion I came to is that due to the bolt being loose, there was play in the lever and the comp tube wasn't closing completlly most of the time (some of the time it was, hence some of the shots were confusingly spot on). This was no doubt meaning the breach seal and barrell were meeting each other with a slight variance, resulting in POI shift.
Long story short, could the breech seals be part of the problem? Or indeed 'play' in the cocking mech.