Microsoft Word file names changing to .~WRD[4 Digits] and then deleting themselves

Ive just finished my PhD part time.For the whole 9 years of my PHD I've been using iMacs and external hard drives (Mac formatted). I always saved any PhD documents to all 3 of my external hard drives AND the iMac. Until my latest recent OS upgrade, I have only had this problem occurring very occasionally. Now it's happening every time Im working on a word document all day, which is 3-4 days per week. As someone said above, Im now regularly looking at the name of the file, and if it changes to a temporary file, then I save with the correct name in one of my spots. Unfortunately I cannot always save on the iMac as we regularly loose power (I'm regional). So instead Im saving to one of my hard drives so if I loose power I can flip to using my laptop. It's **bleep** annoying problem.