First, if your BIOS supports it, enable SMART monitoring for your HDDs. For reasons I may never fathom, most motherboards come with this option disabled by default.

Then, a cheap and easy thing to do, is replace the IDE ribbon cable. Should cost less than $5, and it's about a 30 second job with virtually no possibility of screwing things up.

If that doesn't help, then your drive is probably going bad, and I hope you backed things up recently because it could just flat out stop working at all any time.