There are a number of things that can cause a TiVo to lockup.

Just to list a few:
Some of the software hacks are known to be an issue
Excessive use of the remote control
Setting up or deleting a lot of season passes in a short period of time

If you are having hard drive problems, you would normally notice a degradation in picture quality first. When playing back you would see a lot of pixelation or freeze frames.

What image are you running, and have you installed any other hacks yourself.

Peter.