VPJ's post in Text block fade error was marked as the answer
Luckily I finally managed to find what was causing one of my text blocks seemed to keep changing size/stretching/contracting, when scrolling the page.
It was to do with the 'Pin' setting in the text block. I just toggled it on and off and the block stopped moving.
This was the only thing keeping me from subscribing to Squarespace with a view to going live with my website.
Pretty disappointing that I didn't get any help with this. But that seems to be the nature of the beast (this forum)