My MarkII began acting up. It was shooting fairly tight groups, but there was a high one and a low one about .75" apart. Did the barrel clean, screw snug up routine and tried to twist the barrel and muzzle piece. I've had both these parts come loose on these older guns. Everything was fine so I pulled the stock for the stock bracket screw check. That's when I saw aluminum filings at the forward trigger guard screw and the lug it goes into. The stock is cracked between the front guard screw hole and the trigger housing opening, but it had seemed fine for a long time. The lug was touching the guard before the stock was snugged up into the stock. It had rubbed through the paint and was wearing off the aluminum. I drilled a .5" hole into the guard using the forward screw hole as a guide. This lets the lug slip up inside the guard and stops the touching. All is now well. Amazing how such a tiny amount of play can cause some big poi shifts. I've seen this in some aftermarket stocks, but never in a factory stock. It must have been the crack to blame.