So did I, tried everything and to no avail even horrible brass was a right pain.
The Chinese quick release part off holders are near on useless without a robust machine. In an M300 when tool grabs you can see toolpost flex visibly with old Eclipse 1/16" blades.
Ok for O ring grooves that's about its limitations. Like it or not its as Matts said you have movement/vibration/flex which could be any or a combination of the following. Spindle bearings, bed play, top slide play, cross slide play and the inevitable bed flex. Even if you locked the bed down it can lift up on rear shear through torque on to cutter.
One thing is to try a revolving centre pushed in, bet it will greatly help by reducing the spindle bearing load and bed flex. If hardly any better look at the beds and slides.
Once sorted that try it on aluminium 6082, flood coolant or not the chips wont escape and bind to tool, no warning bang goes an expensive Arno and Iscar tool and tip. Less tendency for it with rear upside down part off 1/8" but still have to be quick to catch when it grabs. Just use rear post as second ganged tool set at 90 degrees permanent.
