2008 Pro, dual 2.8ghz quad cores. Having an issue that I'm not certain what's causing it, hopefully someone can help me get started resolving this issue. I can turn it on, it boots fine and makes it to the desktop. Within a few seconds, the screen goes black, it actually stops producing video and the monitor goes into sleep mode. Within a few more seconds, every fan in the Mac goes wide open. I can usually hold the power, shut it down, restart it immediately and it will usually work for an undetermined time after that. Sometimes, it boots to the desktop without issue. I can put it to sleep or let it go to sleep on it's own, and have no issues. But sometimes when it goes to sleep on it's own, it'll do the exact same issue. Just all of a sudden the fans take off into high gear and there is no waking it or video. Other times it'll do it right in the middle of whatever I'm doing, YouTube or browsing or whatever. I thought it was the nVidia 8800GT since those cards are problematic, so I put a 5770 in it, same thing. Both cards are PC cards, if that matters. I've tried flashing the 5770 but it always takes the flash fine but has a scrambled Apple logo screen and sends the computer into the fan wide open state. Flash it back to the PC version, and it works fine except no boot screen. So I even tried a EVGA 640GT, same thing with all three video cards and that kinda makes me think it's not a video card issue. I've tried several combos with the RAM, nothing changes. Also tried different hard drive, unhooking the dual DVD drives. The Pro is like new on the inside, clean as a pin. I've removed the heat sinks, the fans, the ram cage, basically the entire inside. I built this last summer and didn't have this issue but now all of a sudden it's happening. So I removed all this again a few days ago, reapplied compound and put it all back together, same thing. I used Tuniq the first time, Arctic Silver the second time. Cleaned both processors and heatsinks with Arcticlean by Arctic Silver before every application. Now... attached is a screenshot of my Temp Gauge readings off my 2006 Pro that I'm typing this from. This computer has the dual 3.0ghz quad cores, running Yosemite, blah blah blah. The CPU core temps are way lower than that of my 2008 Pro. I can't get it to boot and stay up long enough right now to get a screenshot of it's temps but I will ASAP. What are the normal temps? Anyone have any idea what's going on? The temps on the 2008 are in the 70's... really high I believe.