Well, sometimes for example, Im doing test renders of some scene or something and like the first 3 or 4 test it renders well, but then runs out of memory and I have to close the Maya, because is like it crash and I have to opened again.
It happends to me always, with diferent scenes.
At the moment, my motorcycle renders well, and also my character, but If i import one of the scenes to the other, I cant render, it goes flushing.
In the help of maya I found this:

Fine displacement critically depends on the specification of a cache size limit, because otherwise the fine tessellation results would not flow through the cache but accumulate until memory runs out. mental ray has a default cache limit of 512 MB.

A good choice is half the amount of physical RAM, or 500-800 MB on 32-bit machines, whichever is smaller. If the number is too large, the operating system may run out of virtual address space; if it is too small, mental ray will perform too many cache flush operations.

.. but It dosent say where I can increment the memory limit.