*** UPDATE - PROBLEM SOLVED ***
Hello everyone,
Firstly, thanks to everyone who chimed in with help and advice. I am really pleased to tell you that I have fixed this issue.
After months of frustration, head scratching and hypothesising, I realised that taking the detailed approach to the problem wasn't getting me anywhere. Based on the comments received, and the solutions I tried, there was no single consistent area that people identified as being the likely cause of the problem, which indicated to me that there was probably a bigger picture in play. Making large changes to voltag