There’s a sentence you don’t read every day. Mks are known to be not quite as accurate as High Standards or Model 41s, but they typically run like champs.
What have you tried? I’d probably soak the whole gun in Ballistol for a few days, let it drip dry, strip it and grease the bolt and recoil spring lightly, and if that failed replace the recoil spring assembly.
I’m a big fan a of the Ballistol bath. My Model 19-3 revolver locked up tight recently. Hammer wouldn’t move at all. I soaked it 24 hours, loosened the side plate screws, gave it a little shake and it started working again. Put the grips back on and it’s dry-firing fine. I hope to get to the range in a few days to test it out.