Freezing and pixellation

I bought a Roamio Pro in December when they were are sale. When I finally set it up in my new bedroom, it wouldn't go past the starting screen, and I had to send it in for a replacement. I just received the replacement yesterday, and there seem to possibly be an issue with the hard drive. While watching live or recorded TV, it continually has problems with the video. If I rewind, and watch the same scene over again, it usually doesn't happen right away again. After some goggling, I found fixmytivo.com. There website describes, word for word, my problem. I ran the hard drive test overnight, but when I woke up, it was on live tv. So I have no idea what happened, or does that itself mean it passed? Do you think I can safely assume its the hard drive, and I am going to have to contact Tivo again?

Thanks for your help guys.