I'm probably a little late to this discussion, but thought I'd throw out the information anyway, especially since "Linc Davis" mentioned failing cables.
First, it does sound like your drive is bad, not a cable. The reason I say this is that if it was a cable problem, unless it was really, really severe, it wouldn't always appear when the unit is starting up.
We use a product called Scannerz which analyzes hard drives and system faults (like cables and logic board cracks.) Info can be obtained from this site:
http://www.scsc-online.com
I've had both a bad drive on one unit, and a bad cable on another. The difference is that a drive with bad sectors on it will always fail whenever the sectors are accessed, whereas bad cables, usually with intermittent connections (think of a loose light bulb) will appear whenever and wherever they care to.
I had a system a long time ago with bad sectors in the OS region. When the system started, it would always either lock up and try incessantly to boot up, or it succeeded the boot process would get screwed up because it was reading bad data. In this case I replaced the disk. Scannerz docs say I probably could have reformatted and zeroed the drive to re-map bad sectors, but I didn't know any better at the time, and it also gave me the opportunity to get a bigger drive (this was a long time ago, and I dumped a 10G drive for a 40G drive - laugh now if you want to but 10 years ago that was a big deal!)
We got Scannerz because they have (or had) a video on their site that showed test results on a unit with a failing cable (can't guarntee it's still there - it was months ago and their web site seems to be undergoing constant modification). We were using a much more expensive tool and it was giving us results that I can only call idiotic. On one test the drive came up with problems at locations A, B, C, and D on the drive, on the next test there were no problems, then the problems were at locations P, Q, R, S, T (the letters represent sector locations). It made no sense. Scannerz apparently targets these types of problems and my company's techs were able to isolate it to a crack in the plastic cable connecting the hard drive to the logic board.
Some people claim it's too hard to use, but I don't see how. Look at the comments on it on CNET for example. One guy says you need to be a genius to use it, another almost implies a 5 year old could use it. The tool is testing only. It doesn't claim to do anything it can't.
