Problems with users being randomly logged out

[JustAskJulie]

I'm running VB 3.07, just upgraded hoping to fix the problems I'm about to describe - but it didn't work. In addition to the forums, I'm running
v3 Articles 1.01
VbAdvanced CMPS
Sigmachat integration
and a custom addition

Within the last week, myself and my users have started experiencing random logouts. Everything seems fine then all the sudden you find that you aren't logged in anymore. This happens most often when switchign between sections of the site - from the forums to the articles, from the vbadvanced home page to the forums, etc - but it also happens just when browsing the forums themselves.

I need any feedback I can get on what might be causing this. I have been using all of the above hacks for monthos without any problems. The problem just started last week, and as I said in an attempt to fix it I went ahead and upgraded to 3.07 from 3.01, and also upgraded/reinstalled (as necessary) the hacks/add-ons. Post upgrade everything is working fine, but we're still getting those random logouts.