problems with nnvirtual and marks

[dme@dme.org]

Recently I've been playing with reading most of my (new) mail/news
through nnvirtual groups.  The behaviour of ticked articles in these
groups seems erratic.

Some times I can enter an nnvirtual group where there should be 2
ticked articles (I ticked them and the group list shows me that there
are two ticked), but none of the ticked articles are listed.  If I
exit the summary the ticked article count in the group list goes to
zero.

In some cases I can now quit and restart gnus and the ticked articles
come back.  Sometimes they don't.

Occasionally things go rather more wild, and a random selection of
articles get ticked, some previously seen articles are marked unseen
(and vice versa).

All in all, a disconcerting experience !

Any suggestions as to how to track down this problem ?  Initially I
thought that perhaps it was because I had:
        (gnus-demon-add-handler 'gnus-demon-scan-news 20 20)
in my .gnus.el (as the corruption somewhat coincided with me leaving
Summary buffers visible when new mail was incorporated).  I turned
that off and things improved for a while, but it's doing it again now.

I tried:
        (setq nnml-marks-is-evil t)
but that didn't seem to have any effect.

Configuration:
        - GNU Emacs 21.1.2 (i386-debian-linux-gnu, X toolkit) of
          2001-10-22 on peppermint
        - Oort Gnus v0.05 (cvs update this morning)
        - mail in nnml groups
        - news on a local nntp server (leafnode).

Any suggestions as to possible causes or how to debug ?  The fact that
I can run for a few days without seeing the problem is making it
frustrating.

dme.