Invalid Node Structure

This is a mid-2011 iMac model. Purchased in June 2011. Started experiencing slow internet on safari and email in Sept.

Ran a Verify Disk and got the Invalid Node Structure..disk needs to be repaired. Ran repair from the disk, told me disk couldn't be repaired, had to erase and install. Brought my data back from time machine...everything worked fine until about a week ago...then same slow down started happening. Ran Verify Disk again, again same problem and procedure.

Now today noticed a slow down in Safari. Ran disk utility and yet again, Invalid node structure occurs, disk needs to be repaired. I just finished the process of getting everything back in order (with the multitude of updates that need to be run!) and am worried that I have to go through it all again.

Something isn't right here...any suggestions on fixing this issue so that it doesn't periodically occur?

Very frustrating...I've had a mac for years, this is the first time I've had one act up this quickly (note; I did take it in to get serviced initially and was told that the hardware was just fine).