What you described is a long established problem on East Coast HSTs, particularly on the bends around Cockburnspath and Grantshouse. Many a time I've seen a coffee cup whizz across the table to the sound of the suspension hitting its limits and we've come close to a spillage. Despite what some might some say, the electric sets don't suffer from this to any like the same extent. So the mk3s are old and unsuited to daily 125mph operation, we get it.
Meanwhile, on Scotrail, the sets will be lucky to get much abve 80 on most routes. Never has my cup whizzed at that speed on any route.
It may be that some on this thread are over excited by the prospect of elderly HSTs providing Scotrail intercity services. But there are also some who are just as overexcited at finding fault, and getting upset about an LNER set rough riding at top speed, and translating that to future Scotrail use, very much fits into that category.