For me this issue has always had a correlation to flash video, and leaving too many windows/tabs open with videos in them. Video of all sorts, really, but mostly flash.
With that in mind, I may have found a fix.
I installed the ClickToFlash extension a day ago, and so far I have not had any forced reloads. I always have tons of windows and tabs open.
The plugin works like this: if possible, Safari will load an HTML5 version of a video instead of a Flash version. If there isn't an HTML5 version available, you click on the video and it plays the flash version. I'm running Safari 6, Mountain Lion, btw.
Generally speaking, browsers prefer HTML5 to Flash b/c it's less of a resource hog. I could go on, but that's really the main issue.
Since installing ClickToFlash, Safari has fun faster, my fan has been less active, and I have not had any forced reloads. There are drawbacks, however.
One drawback is that your videos will play in HTML5, so you don't get certain playback features found in flash players. For most of us, what that means is all YouTube videos, whether viewed on youtube.com or elsewhere, will only have basic play/pause playback functionality. Things like playback speed, sharing, resolution quality, etc., won't be available.
Personally, of the missing features I only care about resolution. However, what I've found is the HTML5 version will typically play the highest res version available, and it will load much faster without slowing your browser or computer down, AND there's no visible reduction in quality. So it's really a push.
One other down side is that some sites will not load any videos if there isn't an HTML5 version of that video available. This can be fixed by going to the ClickToFlash settings > General > check: "Load Flash if HTML5 Conversion Fails". Just remember, if you do that then you have to be careful not to leave that window/tab open too long or you may cause another forced reload!
That's it. Hope this helps.
-Jeremy