Do you have catalog backups? (I just remembered that I haven’t backed up very recently, so I went and did it while I’m thinking about it. 🙁 )
A mere 122 GB of pictures shouldn’t bog down a catalog unless you have an under-powered machine… I have 6,000-7,000 (15-25MB each) photos in my catalog, and I’ve never seen anything like you’re seeing. There are lots and lots of people who have catalogs with over 100,000 photos, so the # of photos on its own shouldn’t be an issue.
It looks to me like the embedded jpg preview inside the RAW files might be getting corrupted somehow. When you edit a photo, it creates a new preview, and the problem goes away.
I would definitely make a backup of the current catalog, then create a new test catalog, and see if the problem persists. If it does, you know it’s not a catalog problem… If it doesn’t, well it doesn’t really prove anything 100%, but I would guess that your catalog might be getting corrupted somehow.
If it’s true that the catalog is your problem, then you’ll definitely want to start a new one right away, so you don’t risk further damaging the old one. After you’ve found out whether your old catalog is to blame or not, you can go back, and try to repair the old one.
I would try deleting the preview cache, and see if that solves the problem. (Be aware that Lightroom will have to re-render all the previews next time you open it, which could take a while for 120 GB worth of photos…) http://www.lightroomfanatic.com/tutorials/advanced/why-and-how-to-clear-your-lightroom-cache/
Let us know how it goes, whether you find a solution or not!