I know this is a bit obvious and I may get laughed at but…do you actually have enough memory?
I was getting a similar problem and I had to solve it by doubling my RAM.
However, the root cause of the problem was definitely Blizz’s unlovely coding by their new generation of interns. For me it happened one day after a patch. The day - and the months and the years - before the patch I had enough memory. The day after, I didn’t. Go figure.
Posted about it at the time and got a generic blue response.
Good luck!