Hi and welcome to OOF

I had this very same problem on my 98 V6 Saloon.I'd only had the car a couple of days when the handling issues started ie. back end going at low speeds at roundabouts and bends also hit some water on motorway(nearly s**t myself) my local garage found no fault,said it was my driving!! No way so I took it for an early MOT where he promptly found that the drivers side rear coil spring was snapped at the bottom!!
Had that fixed and now the handling is spot on in all driving conditions

Before I knew what it was I was thinking of getting rid it scared me that much!! 18 years of driving with no at fault prangs so I knew it was'nt me.
A quick way to check is to run your finger around the coil all the way to check as they are hard to see where they're seated.
I'm not saying yours is the same problem,just worth a check
