Powermac G5 dual 2.5 issues!

I purchased a pre-owned Powermac G5 Dual 2.5 ghz. It has 4gb ram and is running Mac OSX 10.5.8. Very shortly after I purchased the computer I had a crash and couldn't get it to start back up. I tried to re-install software and it would go part of the way thru and crash on the install every time. I started the machine in target mode and hooked it to my old trusty G4. I was able to install the software that way with no issues. I then went back to the G5 and tried installing updates, but part of the way thru the updates it would crash saying the update was possibly corrupt, ( these were straight from apple so I doubt that ) thus leaving my g5 unable to restart because of the partial software update. I did the target mode again, this time installing all of the updates using my G4. After all was said and done, I was back to where i had started when i bought it. I now keep to be getting freeze issue with the machine. I can be online using safari and all of a sudden I get the spinning color wheel. When this happens I can not even force quit the app, it will just sit there spinning away. Tonight i tried opening firefox and it started up and ran for a few minutes before freezing also. A few days ago I had tried to burn a dvd using toast, and that app also froze up and would not let me force quit. I have not had any issues like this from my G4, and after trying resetting the SMU, PRAM and many many hard restarts, I am no closer to knowing what the issue could be. Any help or ideas for me to try would be much appreciated. Thanks, Mike