Macbook Pro Problem: System freezing with staggered diagonal lines

Question

I started getting the diagonal lines about a month ago, exactly as pictured at http://www.maclife.com/forums/topic/106465 (where I have also posted this info) , and also with a total system freeze. Restarts/reboots did nothing- sometimes I could boot from DVD but lines would appear, then finally I was able to get the DVD to work enough to do a clean install of OS X 10.5 after backing up data via target disk.
While I have a read a lot about the NVidia chip issue possibly causing problems like this, my chip is ATI.
I bought this computer less than a year ago as factory refurb off of ebay, and was shocked right away at how hot it ran. iStatPro would show the hottest temps approaching 200F. A week ago, I removed both my 1gb memory sticks and restarted with each alone- one had immediate diagonal lines, and the other booted up perfectly. So- bad memory- fine and easy, just buy new memory, but just two days ago the lines came back. Just for kicks I removed the memory and put the 'bad' one in and the system ran flawlessly. (scratches head)
Ran for a while, until in the middle of browsing through Aperture, the lines came back. Shut down, switch memory sticks again, and everything is fine again. I began to suspect temperature was the culprit- memory gets hot and stops working and put in a cold one and everything works. I've downloaded HardwareMonitor and SMC fan control and at this time am experimenting with fans speeds and running temps to see if a threshold starts to show up. Today I had the fans running at 2000rpm, and had both memory sticks in, but then got the lines (again while browsing safari and aperture open). I now have the fans at 5000 rpm with one stick of memory in.
One other thing I've noticed is that the memory controller was approaching 140F at times with the power cord plugged in and charging, but would drop immediately by 30+ degrees when I disconnected the power cord. Is that normal?
Anyway- hoping my info will give someone more technically informed than me an 'aha!' moment. Hopefully we can figure something out- I don't have Applecare!

Answer 1

Hi Shawn,
The image that you linked to seems to have disappeared, I'm afraid.
I'm afraid just because you have an ATI chip doesn't mean that you can discount the possibility of a graphics hardware issue. ATI chips fail too, on occasions of course. There are other hardware possibilities with such things as well, of course. cables, inverters, etc etc.
Not sure what you were doing when you experienced that 200ºF (or which particular temp you are referring to) but it is certainly higher than you would like under normal operation. The trouble with using things like SMCfan control in such situations is that it can mask other, underlying, issues that really need to be addressed like sensor problems or ventilation obstructions. Potentially a bit like taking an asprin to deal with a brain tumor!
Personally I think you should take your Mac in to a service centre and see if the root cause of your issues can be tracked down, rather than using temporary ad hoc fixes.
Cheers and good luck
Rod