CGS Mod 9 throwing flyers a few FEET off target.

I've been shooting off of a rest, with a red dot, and making sure everything is snug between shots. This thing is all over the place. Probably an 18"-24" group at 10 yards. I'll occasionally get a nice 3" group of 3 or 4 shots and then ZINGGG...I'm shooting a 2 foot smiley face all over the target.

Other times, it's just randomly all over the place and I can barely keep it on a pizza box at 25'. WTF?

I read somewhere that a .45 caliber end cap can help this, but it's louder. Also that it could be the piston, and it wouldn't be such an issue with a fixed barrel spacer on a PCC. Anyone know what I can do?

$600, 9 months, two cases of 9mm subs. Wasted?