CS6 is running out of memory and Purge memory doesn't work

Hello,

I have a problem with AE cache: I render several complex shots with 2 percomposes and different effects on it in a fullHD, quicktime .mov output. I have the following memory preferences,

but After Effects uses more than 6GB, it takes practically all available RAM and then crashes with lack of memory error.

I tried to purge image cache several times but it doesn't work at all. Related themes of a forum gave no answer. AE doesn't want to give the memory back to the system. Is there any way to fix this behaviour without reopening After Effects? I also wonder what may cause such a behaviour: a bad After Effects memory management or a problem with plug-ins I use (memory leaks e.t.c)?

Thanks, Mary.