[Zope] Session Variables disappear

Zope 2.6.2b3, Py 2.1.3, Linux We use session variables everywhere and have a fairly large number of concurrent sessions. Every so often the session variables go away and the whole house of cards collapses becasuse nearly everything depends upon session variables. I tried a while back to isolate the problem without success. At that time someone suggestedit was a known problem open problem. I've just checked the collector and did not see any open issues there. The frustrating thing is that it is the irreproducable Heissenbug behavior. Can anyone shed additional light on the issue. Is anyone else observing similar behavior? Or is the problem something due to the way we are using session variables. -d