Why ubuntu freezes?

[compdoc]

>No proprietary drivers being used. However there is this option at system
settings for ATI/AMD FGLRX proprietary graphic drivers.

>It may freeze several times a day!
>I'd greatly thank for any help!

If there is an option in the mainboard's bios named 'PCI Latency Timer', try
setting its value to 32.

Also, check the cpu and video card temperatures when this is happening. Make
sure all your fans are working. (cpu, video, psu, case) 

Run memtest86 on it for several passes. Let it run overnight if possible.
Make sure it isn't overheating before doing this.

Check the health of your hard drive with the gnome disk utility. This reads
the drive's SMART info.