Well it could be that the drive is getting bad or the lense got dirty but before returning/replacing it there are a few things you can still try.
- Check you IDE cables - try moving the drive to a different position (primary/secondary - master/slave)
- Update your IDE/chipset drivers
- Update the drive's firmware
- If you have installed 3rd party ASPI, check if the layer is properly installed
Finally if it is possible move the drive to a different computer to check if it exhibits the same behaviour in another system. If so, and the guarantee has already expired you can always try to clean the lense. If its still under guarantee simply return it.