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The problem also occurs in rare cases under Windows XP with
Python 2.3.4.  I Suspect the code line

recv_size = max(self._rbufsize, left)

in socket.py to be a part of the problem.
 
In the case that I investigated, this caused >600 allocations
of up to 5 MBytes (which came in 8 KB packets). 

Sure, the memory allocator should be able to handle this in
_socket.recv (first it allocates the X MBytes buffer, which
is later
resized with _PyString_Resize), but it I think the correct
line in socket.py
is 

recv_size = min(self._rbufsize, left).

At least, after this my problem was gone.