Sounds like a failing hard drive if it's hard to access in target mode. From target mode, back up your user data. Run Disk Utilities and if you can't repair it, reformat and reinstall the system from the Mac Mini if it is not a Intel processor machine. That failing, replace the hard drive. Here's how to do it:[guide|345]
Here's some hard drives that will work for you: http://www.ifixit.com/Mac-Parts/PowerBook-G4-Aluminum-15%22-1.5-1.67-GHz#0-Hard+Drives+PATA+