This is exactly the same behavior I was asking about in this thread.

There's a time limit set on the board that detects activity on a search. If you dally too long in a thread (which is easy to do these days as I'm reading a lot more than I have in the past, or you follow crosslinks to other threads) then it deletes your search and you lose the bolding and list of new posts. It's very annoying.

I've started recently reading another board that uses this same software, and it does the same thing. The explanation I recieved from the admin there was that it drops searches after seeing no activity to save memory and bandwidth on the server. His suggestion was to bookmark long threads and come back them later after you read all your other new posts. It's annoying and cumbersome.

Which brings me back to question of Dan I had a while ago - is there anyway to extend the time period allowed before you lose the search?