zombie session information?

My apologies if this is covered somewhere else.  A googling and search here came up with too many unrelated hits.

I've got an issue where session data comes back from the dead.

I'm using 3.0.5 with file based sessions and saving the fact when a user logs out by unsetting a session variable.

I've added all sorts of logging and can clearly see that the session var is cleared and right after the next redirect, the variable is set again to what it used to be.

Is there something I've missed?  Forcing the persistence before a redirect?

One other detail...

If I delete all the session files, things work as expected for about 3 iterations (of log in log out) and then stops working again.