St Malverns Dance is a wobble that usually occurs at around 50 mph even in a straight line.
I recognise what you have experienced, as I am sure many others have on occasion. You can throw money at it with BRBS, shockers, etc but personally I think it is a Morgan “thing”. The relatively firm suspension meeting usually, a bumpy corner or one with poor camber and maybe too much throttle at the wrong moment, can easily upset balance.
It is worth checking the basics but if the tracking is incorrect, you should see it as uneven tyre wear. Too much toe-in will wear the outers of the tyres whereas too much toe-out, will scrub the inners.
