The PSU fan in my 64-bit Ubuntu work machine ran a bearing and I replaced it earlier in the week. The original fan had its leads soldered directly onto the PSU circuit board, as I was being lazy, I just didn't find a soldering iron and connect the new one in the same manner but rather plugged it into the chassis fan connector on the motherboard. I installed a "widget" to monitor the temperatures inside the case as I'm not sure if the M/B steps the case fan-speed down or not...
I received GPU temperature warnings for about 30 minutes at a time and then everything settled down only for the process to be repeated an hour or two later. I eventually tracked the problem down. It only happens when a "Proth Prime Search (Sieve) 1.21 (cuda23)" workunit is running. The GT9800 runs at around 63'C for SETI & Collatz. As I type this, the GPU is reporting 72'C on a PrimeGrid workunit.
I'm a bit concerned about the life-span of the GPU at these temperatures (as well as the irritating fan noise), so I have changed my preferences to not accept GPU work from PrimeGrid.