mac pro 8-core fan problem

Hello -

On my Mac Pro, all the fans run real high sometimes when I turn it on, sometimes it kicks me off Safari, and sometimes just the monitor shuts off. Maybe one of these 3 things happen once or twice a week. When it happens I hold the power button down on the tower to shut it off, and then I turn it back on, and it will be fine. Don't know if these are related issues or not. Any ideas or advice appreciated.

Mac Pro 8-core 2,1 3Ghz OS X 10.6.8

2GB memory, 667Mhz DDR2 FB-DIMM

500gb HDD

30" Mac Monitor