It happens about every six months when I am generally flying. Flying KJFK to Philadelphia International (basically the reverse) is fine. The last time this happened was bush flying in Alaska around Tongass after 4 hours or so.
One other area of the sim I have more than a few add-ons close together is London with Aerosoft Heathrow and the London city scenery by Aerosoft also (iirc). I had an OOM there once in the PMDG 747, back when I was on Win7 64b RC installed over Vista 64b. I think it happened around LFPG once back then also. These incidents were not repeatable and I fly in and out of EGLL, LFPG, KJFK regularly both CA and GA.
Other than that my OOM problem is pretty rare.