My 612 wouldn't freeze that often but it did happen every now and then. The big problem I had with my 612 was that it kept loosing my sat signal, even when my other receivers would not. The "acquiring sat signal" message would come up, I'd hit the guide button and the signal would return. Few minutes later the same thing would happen again, and again, and again, and again. Rebooting the box would cure it for about a week and then the whole process would start over again.
As I'm typing this a tech is hear putting in a new 722 box so, I'm kicking the 612 to the curb. For what it is worth, the tech said that the 612s are known for being buggy.