Another "Premature end of script" headers problem

["Dan Yamins" <dyamins@xxxxxxxxx>]

Sorry, I posted this in the middle of someone else's question before ... so I'm posting it again in its own thread.  Hopefully someone has some ideas:

I'm running a python script using CGI on a local apache-served virtual
host. (It' just an internal development project.)   The project
involves layout of some large graph network structures, which are then
viewed via a web-based graph-vieweing tool.  The script uses the
"Pygraphviz" module (http://networkx.lanl.gov/pygraphviz/) 
to generate .svg files with layouts of the graph and the "zgrviewer"
applet to view the graphs.    During the script, I first construct the
graph.  After using the pygraphviz module to do the layout of the graph
as a data structure in memory, I invoke the pygraphviz method "draw",
which saves the graph to an .svg on disk.  Then I issue the HTML code
invoking the zgrviewer applet, which uses the saved .svg file.

Sometimes -- only _sometimes_ -- when the script invokes the "draw"
method, I get a "premature end of script headers" problem, with error
code 500.    Because the .draw method is outputing a file to disk, it
seems like it might be a permissions-related problem. But I can't
figure out what t is easily, becuase the problem only happens
occasionally.  Moreover, when it happens, if I just hit "refresh" on
the browser to run the script again, then it usually runs fine.    I
haven't been able to figure out more than this. 

Any suggestions greatly appreciated!
Thanks,
Dan