Imac keeps freezing up

Just bought a imac 20 inch silver 4 gig ram operating yosemite, when i plug in the ethernet surf for 10 mins then it freezes up and i have to unplug it. then i go to disc utilities and something in the storage folder is messed up, so i repair it and same thing happens, dont know what it is. also the yosemite is in the storage folder. any help would be appreciated, thx

What storage folder? Do you mean the hard drive? Did you buy this new?
If not, did you reinstall the operating system? You should.

The last 20" iMac is now over six years old so suggest you go into Recovery Mode and run repair Disk over the hard drive and see what is reported.

We'll need more details on exactly what 20-inch iMac you have there, but my first suggestion would be to sign in to the Mac App Store under your own account, download the Yosemite installer, create a bootable USB 3.0 flash drive of it using Discmaker X, then use that to completely wipe and re-install Yosemite from scratch. That should fix any issues, and will ensure that this machine has not inherited the problems that made the previous owner sell.