OK,,this is difficult to explain, but I did a track day the other day,at lime rock and coming off the downhill into the right that leads to the high speed straight away, downhill is 85mph to the straight which topped out at 110mph, when the car would level out . The reared of the car would float ( or rock) left and right like a boat, then settle in and perform as directed.
This car is a 99, with 80k on it, it is box stock, with z rated 18" tires. Except for an aftermarket nose that resembles a GT3 front end.
It only does it on this one part of this specific track. I am assuming the spoiler is up at that speed.
Any ideas, the instructors said he's never seen it before and has been in cars with older vintage and more miles,,,other than struts