Firstly, take a deep breath.  This probably isn't going to be easy.  This seems like memory corruption, possibly due to threads.  That's a guess, since I don't know if you are running with threads.

The first (probably faster alternative) is to build python and all the C extension modules --with-pydebug (passed when you run ./configure).  That might catch the error slightly earlier.  It won't necessarily point to the cause of the problem, but may help narrow down the potential causes.

The second alternative is to try running this under valgrind or purify or with some memory debugger.  It seems like there is memory corruption (I'm guessing from mod_python).  This will be more robust at finding the problem nearer to the root cause.  Unfortunately, it might prove too slow to be useful or find the problem.

If you can narrow down the test case that would help a lot.  For example, could you eliminate subversion (presumably using the C extension module) or mod_python?  If you could eliminate one and remove the problem, that would help a lot.