I know lube can be a benefit - but it can also be a big cause of issues. I would remove all the lube, clean the liner thoroughly and shoot a few groups. Too much can cause a pressure wave with the shot. A little always to start with.

Check the obvious stuff - not clipping in any way any moderator, check the pellet probe is tight and Chrono the shots.

The power your getting should be enough.......it is also possible your liner just does not like that slug.
217 or 218? I think FX recommend a 217.....

No magic wand from me, just trial and error, try a lighter slug, see if that suddenly tightens everything up.

G