Python not freeing memory (?)

[Chris]

I have a Bayesian simulation package that keeps running into memory 
allocation problems. I have a feeling this has something to do with 
Python (2.5.1.1) not freeing memory. The program essentially 
iterates n times, each time proposing new arrays (numpy) of values 
that are evaluated using statistical likelihoods. All variables in 
the loop are re-assigned at each iteration, so there should not be 
a leak. Nevertheless, at approximately the same iteration every 
time, I run into malloc errors:

Iteration 36200 at 5647.58165097

Iteration 36300 at 5664.8412981

Iteration 36400 at 5681.71009493
Python(28344,0xa000d000) malloc: *** vm_allocate(size=8421376) 
failed (error code=3)
Python(28344,0xa000d000) malloc: *** error: can't allocate region
Python(28344,0xa000d000) malloc: *** set a breakpoint in szone_error 
to debug
Traceback (most recent call last):
  File "/Users/chris/EURING/detection/detection2.py", line 285, 
in <module>
    run()
  File "/Users/chris/EURING/detection/detection2.py", line 268, in run
    results = sampler.sample(iterations=iterations, burn=burn, thin=thin)
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-
packages/PyMC/MCMC.py", line 3021, in sample
    parameter.propose(debug)
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-
packages/PyMC/MCMC.py", line 768, in propose
    if not self._sampler.test():
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-
packages/PyMC/MCMC.py", line 2899, in test
    self()
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-
packages/PyMC/MCMC.py", line 2562, in __call__
    return self.model()
  File "/Users/chris/EURING/detection/detection2.py", line 238, in model
    self.pd = invlogit(self.beta0 + self.beta1 * self.wind + self.beta2 * (self.cloud>0) + beta3[self.series-
1])
MemoryError

Is there *any* way of getting around this?