to be honest m8, I would look to get a replacement, my old drive did exactly teh same thing and I got it RMA`d no problem, if u have had it 20 months I think its unlikely its under warranty.
when you delete the controllers and restart it should reinstall them at the level the HD should operate at in your case probably DMA 5. It is possible that it is repoting so many CRC`s that it is dropping to DMA 2 straight away. If it is making a clunking noise I would back all your data up right now becase its a sure fire sign its about to fail.
remember hard disks are mechanical devices its not a matter of if they fail but when. fortunatley for you HDD`s are very very very cheap atm and you can pick up a 120gb drive for around £70.
couple of weeks ago I got a brand new 160gb 8mb cache 7200 maxtor for £100, that is a lot of storage for your money