Python not giving free memory back to the os get's me in real problems ...

[Paul McGuire]

You might try looking at references between objects, especially if
there are any cyclic refs.  For instance, if you have a data structure
in which child nodes have back refs to their parents, try changing
these to use weakref's.  This may help the garbage collector to better
reclaim discarded objects, so that not so many will need to be held in
memory.

-- Paul