|"I know that the motherboard is good and the RAM is fine because Linux is working perfect"|
Doesn't mean it's so. You should always test your RAM with memtest86 before putting it into service.
Looking at what few specs you provided, I know your CPU is not 2.8 because the fastest socket A was only 2.2GHz. I assume you have a 2800+ which runs at 333MHz FSB (166MHz frequency)? If so, dial down your RAM speed to DDR333 (166MHz frequency) to match the CPU. With a socket A system, anything other than 1:1 ratio hurts performance.
Good board though...make sure to tweak your BIOS settings.