Why does OS X sometimes crash on startup and shutdown?

It doesn't happen all the time. Have run disc repair from install disc. One of the letters "m" has to be manipuated to work. Movie downloads sometimes freeze or crash on iTunes. Took it into Apple and tech looked at the data history and said it was probably a hard drive issue. But, I here no clicking in the drive. I have everything backed up. Just wanted to make sure there was something else I could try before I put in a new hard drive. Re-install O SX maybe? Thanks. Mike