I had a similar problem that I resolved myself.... I think it was partially my code hogging too much memory while too many "big things"
in my application - the workbook goes out and grabs another departments "daily report".. and I extract out all the information our team needs (to minimize mistakes and data entry).
I pull in their sheets directly... but I hate the fact that they use Merged cells... which I get rid of (ie unmerge, then find the resulting blank cells, and fill with the values from above)
I made my problem go away by
a)unmerging only the "used cells" - rather than merely attempting to do entire column... ie finding the last used row in the column, and unmerging only this range (there is literally 1000s of rows on each of the sheet I grab)
b) Knowing that the undo only looks after the last ~16 events... between each "unmerge" - i put 15 events which clear out what is stored in the "undo" to minimize the amount of memory held up (ie go to some cell with data in it.. and copy// paste special value... I was GUESSING that the accumulated sum of 30sheets each with 3 columns worth of data might be taxing memory set as side for undoing
Yes it doesn't allow for any chance of an Undo... but the entire purpose is to purge the old information and pull in the new time sensitive data for analysis so it wasn't an issue
Sound corny - but my problem went away