I've never had stability issues with my M6 on stock tires at any speed up to nearly 150 MPH. It's always been rock solid.
I did feel a bit squirely with the new DRs on the big end at the drag strip but I attributed that to the wider sidewalls, low tire pressure, and cross-winds that day.
I put the car up on a lift and do a safety inspection before each track visit. So far no loose components. Still, as others have suggested, there is a bulletin out about loosening struts (
ZL1 Front Struts FYI). This may not be detectable by hand on a lift.
I don't see how old differential fluid would cause the behavior you describe but I do change mine after every 1 or 2 drag strip trips, depending on how hard it's driven. Faulty eLSD could potentially cause instability if it clamped down with too much pressure or at the wrong time.
travislambert has opened this thread to investigate eLSD issues with the M6:
eLSD Wheel Slip % on M6
I hope this gets resolved soon. I'd be interested to know what's causing it too. I can imagine how frustrating it is.
--Cal