So you had a bad hard drive, sent it off for RMA, got one back. Do you know if it was the same one you got back? did you check the serial numbers?
The "lagging" is mostly a trait caused by a failing hardrive or not enough power being supplied. Not enough power being supplied to the hard drive can cause this "lagging".
You say it lags every 15-20 seconds. What happens? How is it laggy?
- do you see the hard drive light go crazy every 15-20 secs
- does it slow down and then speed up and then slow down again?
Not that I can remember, I had to send it off to get a new hard drive, when I did that i just installed an un-tainted with marketing bullcrap version straight from microsoft.