iv'e been shooting one of these for over twelve months now & had some accuracy problems. however its pretty good now. You didn't mention 4.53's for your pellet testing, i'd give them a go if i was you first of all. Secondly, clean the barrel. But most important check the barrel shroud, which the stripper joins on to & then screws on to the barrel is clear of the air cylinder. mine was'nt & was pressing down on the A Frame. it's only a small air cylinder on the mk4 & 200 bar is a lot of pressure, mine would not stay on zero if i put to much air in(you can safely charge to 232 bar). I now charge to 195 bar max and have adjusted the shroud so it,s clear of the cylinder, it's improved considerably.
Even if you think your on the right road, you'll get run over if you just sit there.