It shouldn't change formats unless you press the "Modern View" link, or unless you have the option set for your forum preference to be "Modern View".
Go back up one level after reading this and look for the Options link in the link bar above the forum post list. If you end up back in modern view when you try to do that, go directly to this link (http://www.yesterdaystractors.com/dbtest/wwwboard1.html). There you should see the Options link. Click that and down toward the bottom, is a check box called "Forum Preference", that should not be checked. Make sure it isn't and if it is, uncheck it and submit the form. Then click return to forum and that behavior should stop.
If that isn't the problem, let me know how you are moving between forums. There is the "Hop to" box on most of the forum pages or there is the link called "All Forums", or lastly clicking on "Discussion Boards" in the left hand menu.
Since you've been going to Modern View when you don't want to, the problem you had there is a real problem. I found it while trying to figure out what you were seeing. The "Classic View" link from the Modern View, is returning people to the wrong place most of the time. We'll figure that out, but since you didn't ever want to be there in the first place, it's not the basic problem you are having.