Macbook Pro (late 2008) freezes randomly, have to reinstall Mavericks to "fix" it
Personally, I'd assume you're looking at the machine either having bad RAM, causing the OS to corrupt and thus forcing the reinstall, or the machine having a bad hard drive, or at least one that's starting to fail.

For testing, I'd download a copy of Memtest86 (the free version) at http://www.memtest86.com/ , follow the directions to install it to a USB drive, and let it run through a few loops to check that the memory is okay.

If that comes back clean, the next culprit would be hard drive.  I'd suggest downloading the free trial version of Drive Genius from http://www.prosofteng.com/downloads/ and running your drive through its tests.  If it is a bad hard drive, replacement is easy enough, there are multiple guides on youtube as well as at iFixit that detail how to take your machine apart and replace the hard drive.  Good luck!