Excel 365 - corrupting workbook constantly and is unstable

JKPieterse 

Hi

Originally the file was xlsb and 100mb-110mb. It died and on recovery I tried using it as xlsm - 140+mb

As this opened once and somehow corrupted itself in that it refused to close and refused to save, I decided to make it into smaller files. I split the file into 5 pieces, keeping the main sections together and and related data tabs together in the smaller files. 

This was done linking via vlookup and sumifs, but even with all the files open at once was slow, and prone to the grey screens (refusing to screen update sheet and even toolbars), corrupting, and occasionally memory error or just closing without warning or saving.

Since then I wrote some vba to grab the data into a tab in the main sheet (crunched/summed to reduce data).

As to your points to check:

There are a few links on sheets that do not matter linked to the original file.
The named ranges are there, I have removed those as they did not pick up the files being split.
Custom styles, two apparently

cleaned vba code?

No worksheets have objects on them (form or shapes). The headers have images on some sheets.