Mask, I assume you have tried to make a new clean install of PST without any luck, right?
There is always the slight possibility there is some problem with your original cd's and the way your drive read them during installation. I would suggest ,if nothing else helps, to make images of your disks in your hard drive using Alchohol120 or cloneCD and then install the game from a virual drive (alchohol or daemon tools).
While not my first attempted solution.. Reinstalling was among the top 5, and sadly, failed to work any wonders. Will give virtual images a shot, but I'm betting against. It'd simply be too easy.
edit: would time of installation matter? I installed all mods after making it past the alley of lingering sighs. Any chance it has something to do with editing previously entered area's in these two cases?