Hi Que, this is a problem with FSX being a 32bit application, so it can only use a maximum of 4GB of memory (virtual address space). When the program reaches this 4GB VAS limit it will cause an out of memory error and stop working.
There are a number of things that increase the risk of OOM errors - running with frames rate limiter set at unlimited, using a setting higher than 4.5 for LOD radius, high levels of AI traffic, using HD textures (4096) for clouds etc.
This is becoming a more common issue in FSX as people have more powerful machines and can run the sim with all the bells and whistles. There is a lot of discussion on this over at Avsim so you might want to have a look over there. You can monitor VAS usage by FSX and see how adjusting your settings helps.
I hope this helps,