[mod_python] stumped by inputfilter infinite repeat

hi, all.  i'm using mod_python with a downloads/uploads handler,
tramline ( http://infrae.com/products/tramline ) with a content
management system (plone).  when the tramline input and output filters
are hooked in (using SetInputFilter and SetOutputFilter) and i visit
the zope/plone site via a redirect, i get infinite repeated calls to
the input filter.  (i can observe the this by setting PythonEnablePdb
- from the perspective of the web browser, no page is returned.)  i
can provide more details, and realize that this may be a tramline
problem, but it seems like a behavior that might be familiar to
mod_python experts, so i thought i'd run it by you all first.

things like mpinfo and mptest.py work fine.  visiting the regular,
file-based mockup site yields no problems - only the embedded zope
site, via a rewrite rule:

  RewriteRule ^/testbed(.*)
http://127.0.0.1:8080/VirtualHostBase/http/%{HTTP_HOST}:80/VirtualHostRoot/_vh_testbed/$1
[L,P]

(the elaborate VirtualHostRoot stuff is for a zope virtual hosting provision.)

does any of this sound familiar to anyone?  suggestions about
debugging it?  (stepping through the input and output filters using
PythonEnablePdb yields little or no illumination - they just keep
getting invoked.  i don't know what to look for tracing added to
importer.py FilterDispatch, which i tried instead of using
PythonEnablePdb.)

any help would be appreciated!
-- 
ken manheimer
http://myriadicity.net