Have you tried replacing the drive cable? It could be that it's faulty. I don't know what drive tests you've run, but you could install smartdisk utilities and see what is reported about the drive.
If you are convinced the drive needs replacing, then replace it with the one you've bought. Ordinarily, I would say use dd to replicate the root drive. But, if it's failing, you are liable to get erratic results. So, here's what you can do. Download this script
Old-Polack wrote which catalogs the packages you have added after PCLinuxOS was installed. Make the file executable, then run it in a terminal by doing:sh find_installed_apps.sh
It will create a text file named installed-packages.txt which will have a list of the extra packages you installed. Keep both files in your home directory. Leave the disk with your home directory where it is. Replace the old root drive with the new one and make sure the jumpers on the drive are correct. Boot the PC from the latest PCLinuxOS live CD. Wipe all partitions off the newly installed drive, then reboot from the CD again. Install the OS and repartition the drive to your liking. During the installation, designate your previous home drive to be mounted as /home, but DO NOT
format the disk with your home directory.
After you have reinstalled PCLinuxOS, immediately do a full update. Follow any instructions given after the update. You can then use the list of your previously installed programs to reinstall them.
If you want to be sure you have a safe backup of all the settings stored in your current home directory, such as .mozilla and .thunderbird folders, do this before you swap any drives. Create a folder in your home directory and name it Backups or Settings or Hidden or anything you like. Set the file browser to show all files and folders. Copy all the hidden files and folders to your newly created folder. Alternatively, you can copy all those hidden files and folders to an external drive, like a USB thumb drive. It never hurts to have backups.
If you have another computer, you could mount your old hard drive there to do further tests. It may be that you just have a faulty cable. Or it may be that your drive is failing, as you believe.
EDIT: If it were me, I would replace the hard drive cable at the same time the new drive is installed.