My Intel iMac 24" (Mid 2007) when I start it up about 3-5 minutes in the fan will start going at full speed. The temps are all good and stay consistent, but the fan still roars. Once it hits a certain point a message will pop up and state to restart the computer. It is only the CPU fan that does it, the other two fans are functioning normal.

I have tried an SMC reset and that hasn't worked. I have tried to run a hardware diagnostics test, but the computer doesn't allow enough time to complete it. I opened it up and blew out all the dust as well. At this point I believe it is a hardware issue I just don't know where to start. If it be the fan or one of the temp censors. Anyone have any recommendations, or ideas?