Len,
Thats why you should be using "New Posts" instead of Recent Activity.
New Posts will not time out the list on you if you take too long. You mark each forum read in the "New Posts" list as you finish with them.
This is as good a time to point out that vBulletin is now preparing a new release for beta test, and in that new release, there is a new way of handling the marking of threads read.
Every thread is tracked in the database as to whether or not you've read it. Any unread thread, or thread with activity since you read it last, shows up on the list. Once you read the thread, when you go back to the list, that thread is gone. Threads will not "fall off" the list until they've reached an age we set (probably 1 week).
We don't know when 3.5.0 will be released, but PoA will be setting up a test site running the beta software as soon as vBulletin puts the beta out there. Hopefully early this month.
The downside to 3.5.0 is that, when we move to it, some features we've added recently will go away until updates are made for them. 3.5.0 changes some fundamental behavior, in a good way, but in a way that doesn't retrofit well.
Anyway, I'm going to continue working on the email system to try and get the daily digest to contain all new posts (and only new posts) from the day. It will take some work tho, so no promises.