Hangs at times. Disk Utility shows different free HDD space.

Running Yosemite with a half terabyte HDD. Laptop sometimes hangs, apparently not heat related. I have tried repairing file permissions to no effect. Tried Repair Disk but no errors found. What Disk Utility did find was that if I highlight the main or root hdd and click Info, it says it has 20 MB free space. If I highlight the partition in it, Info says there's 200 GB free space. So obviously there's a foulup in the reporting between the two. Maybe this has something to do with the intermittent hanging. How do I fix this?