24' 2.16ghz video issues and freezing w 10.5

I was running 10.5.x - I honestly forget where it was at version wise but it was fully updated as of Thursday the 25th of Feb. and I was getting many random horizontal "rainbow" lines in Safari (also updated to that day) and also the machine would totally lock up and need to be powered off via the power button and restarted. Also, I have an AOC 22" monitor on extended desktop and it would randomly dim like it was going to sleep and then wake up on it's own while I was working on it. I had a 1tb WD drive for almost 8 months and a fresh install as of Monday the 22nd of Feb so I doubt it was that but on a guess I swapped in my OEM 250gb drive with 10.4.11 on it and have had no issues since. I installed the widget for monitoring temps and my GPU temp with 10.5 was almost 62* C and running the same apps in 10.4 it is 50-51* C so maybe that is my issue? I was told to take it for repairs to an authorized Apple repair center BUT I can see this taking weeks to sort out and I have to run my whole life from this computer as I sell on Ebay and dread going back to Windows even for a short time. Anyone else experience this with this particular machine in 10.5? I was running 10.5 since I put in the 1tb drive with no issues up until then other than occasional horizontal lines in Safari, also I when it started locking up I was getting the same lines only thicker when playing a DVD as well. I do not push this machine as normally ALL I have running is Mail, Safari, Firefox and sometimes GarageSale and Iphoto but very rarely more than 3 of the above at one time. No gaming etc either.