The only thing sure, it's that is not related in any way to PHNL.
A scenery can't cause a memory leak in FSX like the only you are describing, and our sceneries even less so, since they are handled externally to FSX by the Couatl.exe module, which means IF there was something wrong with the scenery, the high cpu usage and memory allocation should be seen in the Couatl.exe module, not in FSX.EXE, there IS a reason why we use an external .EXE, so it won't take any memory from FSX for its own logic, and it won't be able to crash FSX too.
Most likely cause, you are using a resource intensive airplane addon or another addon with an in-process module running inside FSX and this, *combined* with the usage of a detailed airport, is exhausting your resources.
The first obvious test, should be using PHNL with a default airplane and notice if the crashes are still there.