At Swindon one of those sleek new Hitachi GWR▸ electrics turned up. ... The ride to Reading was dreadful.
Ouch. That sounds very much like the exception; I'm a passenger and don't know the technical stuff but I have made dozens of part-route trips London - Chippenham - Bristol and London - Cardiff - Swansea services and also a North Cotswold journey and the ride has been good in each case.
Possibly deflated suspension bags on that bogie? Did you try another carriage? A train can proceed as such perfectly safely at line speed though the ride feels extremely rough. The IET▸ ride quality under normal circumstances compares with a HST▸ in my experience.
Thank you for that re-assurance ... interesting to hear if others have had rough journeys?