I had an issue with my Intel Mac Mini randomly rebooting last week. It always seemed to be when I was watching a video. I thought it was my video card again, so I rebooted and reset my PRAM. (reboot holding CMD + OPTION + P + R until you hear the boot chime a 2nd time) I had to do this when I upgraded my monitor from a 19” to a 24”, and went from a screen resolution of 1280×1024 to 1920×1200. But this did not fix my issue. What really annoyed me was that my screen saver (Electric Sheep) would throw artifacts and then freeze the entire machine. After 2 days of trying to trouble shoot this, I finally had a new symptom to look at – my external HD, which holds my music collection in one partition and my Time Machine backups in another, would not re-mount. After some fooling around, I finally got my music partition to show up by not my Time Machine partition.
I went into the Utilities folder in OS X and ran Disk Utility. I tried to verify my external HD but it would not verify my Time Machine partition. I then tried to fix the drive, and again, it failed to recognize my Time Machine partition. Seeing this many times before, I knew the drive was in the throws of failure. I turn off the drive via its external enclosure and moved on with my day.
However, something else as bothering me about the situation. I wasn’t convinced 100% that my video problem were caused by the external HD. I knew after seeing the drive fail diagnostics that this was the cause for my random reboots. But, I was unconvinced that the drive would cause my machine to reboot during video playback or make my screen saver freeze. In my research I had come across a lot of articles about machines overheating, especially during heavy loads. I never thought of playing a few YouTube videos has being a ‘heavy load’ so I never gave it a second thought. Not content with blaming the HD for everything, I went back to those articles and did some more research.
The first thing I noticed was that most articles stated that their Mac Minis ran idle at a temperature of about 50-55C. Idle, my machine was running at 67C. I wasn’t sure why mine was running so hot. I even have the Mini sitting on its side so the air vents are 100% unobstructed. Doing some more reading on the subject, I came across a few articles on fan speed. I’ve never used a fan speed control program on my Mac. Looking at my fan speed via iStat Pro, it was running at a steady 1500RPM. Most people recommended running the internal fan at a minimum of 2000RPM. Trying out several recommended free programs, I liked one called Fan Control the best. It gave me all the options I wanted and was quite transparent. I set the minimum fan speed to 2000RPM while the CPU is running above 50C, and set the maximum temperature I will allow the system to run at to 80C. 
This is well under the specs given by Intel for my processor, but better safe than sorry in my book. After about 10 minutes with the new Fan Control setting, my Mac’s CPU temperature was down to 64C under normal load. Letting the system sit idle for about 20 minutes with no programs running yielded amazing results. My Mac was now running at a steady 44C-46C idle temperature. I was very happy about this. I never thought a little free utility program would make such a difference. 
For the record, here are the stats of my Mac Mini.
BTW, I ordered a replacement external HD as well. A Western Digital Elements 1TB external HD is on its way yo me as we speak.
