Premiere's handling of Render Files

Can someone elaborate on how Premiere handles render files? I can not figure it out. If I render a sequence, and then make a change in that sequence to a clip, unrelated clips then become unrendered, and I've not been able to figure out any rhyme or reason. It seems often times, that what needs to be re-rendered is random, although I'm sure there's a connection I'm unaware of. Still, quite frustrating to have to re-render all, or big portions of a sequence after changing only one clip.

As well, if I render a sequence, close the project, and then re-open the project, then, again, the sequence needs re-rendering. Anyone have any advice or light they can shed on this situation? It's making my work go painstakingly slow.

Much thanks!

I'm running Premiere CS 5 (i'm exporting right now, so can check the sub-version number, but I update regularly), on a 2011 17" Macbook Pro 2.2ghz core i7 with 8GB of RAM on OSX 10.7.2. Everything is running off of internal drives, so there shouldn't be a risk of a drive disconnect issue.

It seems that changes don't affect random clips, but rather all the clips in a track. So that a change of a clip will require re-rendering of every clip in that track. Is that how things work? Even still, it does not explain why my render files would disappear open re-opening a project.

Is there any other info you might need? Thanks for your help.