I don’t know the WT decks from back then. What is the belt made of, I assume it’s not like the current WT decks’.
I had a similar problem with a Townshend Rock Mk2, which uses a round rubber belt, basically a 1/16” cross section O ring. After fitting a new belt there was speed instability noticeable on piano music. The belt was taking a set when it cooled after use. A belt direct from Townshend was the same so I ended up going back to a 10 year old belt. I’ve since replaced the Rock as my main deck but it still gets occasional use for mono, I remove the belt from the pulley when I stop it for longer than the time it takes to clamp another disc.
I had a similar problem with a Townshend Rock Mk2, which uses a round rubber belt, basically a 1/16” cross section O ring. After fitting a new belt there was speed instability noticeable on piano music. The belt was taking a set when it cooled after use. A belt direct from Townshend was the same so I ended up going back to a 10 year old belt. I’ve since replaced the Rock as my main deck but it still gets occasional use for mono, I remove the belt from the pulley when I stop it for longer than the time it takes to clamp another disc.