This happens when: You change from the Word window and back,

That would be pretty normal (since, if the document is large, Word needs to
swap it back into memory, which takes time and ressources).

when something is typed *and* the cursor is moved, when you
backspace to delete.

Do you have any add-ins loaded that might do something in the background?
You see COM-Add-Ins in the Help menu Info System Info Office Word
COM-Add-Ins

Another thing that might help a bit would be to work in Normal view and to
turn off background pagination (Tools Options General).

Regards,
Klaus