Possible rear suspension issue - unstable at higher speeds

BGTV8

If rear toe is toe-out, get it reset to factory spec's - a bit of rear toe-in.

If still a problem, then something has to be loose .... you have rear camber arms and toe bolts, so check that the toe bolts are not loose - they might be tight on the spanner, but there may still be a degree of freedom under lateral suspension loads so look carefuilly for shiny metal indicating that even though the bolts are tight, something is moving.

You are decribing steering from the rear which is really unsettling to ay driver - is usually caused by toe-out at rear, but could be loose suspension as well.

If the traction link has a bush that is flogged out, that may provide enought movement to give you some dynamic toe-out at the rear and that will always give a driver a does of the squirts ...

Put the car up in the air (axle stands or hoist) and go over it closely with the Mark I Eyeball and a proper spanner (wrench) check - that answer is in the car somewhere.

RB