I have the same problem on The Riesel Problem (Sieve). 24GB physical RAM, 16 cores running, and roughly 50% of the memory used.
I noticed that under preferences, it said use only 50% memory when the computer is in use. I changed that to 90%, but still the same problem. I did not restart BOINC after changing, but I did so right now, and will get back to you if that helped :)