Sorry Sami, I hadn't seen this when you first posted about it.
When you click the 'view posts since last visit' link, it performs a search based on what's been posted since your last visit, and displays the results. It looks like every time you are going back to that results page, it is performing a new search to include the very latest results, which is strange - for me, it just displays the same results as last time, ie it doesn't do a new search and it just shows me the same results as when I first clicked on 'view posts since last visit'.
You're not clicking on the 'refresh screen' button in your browser at any time, are you? This would perform a new search, and would activate a delay warning if it was too soon after a previous search.
Whilst I'm not sure why this is happening for you, one way you could get round this is to hold down the 'shift' key when you click to open a thread - this will open the thread in a new browser window, which will allow your page of results to remain untouched in the window behind. When you've finished reading the thread, close that browser window down with the 'X' and your original results page will be displayed for you to select a new thread from (again using the 'shift' key if you want to).
Meanwhile, just to let you know, the search delay is set to something like 20 or 30 seconds - this is in place to reduce load on the database at any one time.
Is anybody else having this problem?