bad checksum message , loss of processor speed and time

i have a home built computer with ecs k7s5a motherboard, 1.3 amd atholon chip , 512meg sdram, nvidia video card with 128 ram , windows xp home . The problem i have is that every several days it won t boot up , gives bad checksum error and i have to go to bios and reset time and also speed of processor to 133x10 . Bios did not autodetect when first built as it should have also. I have tried 3 different NEW batteries also. Does this about every 7th or 8th time you boot. Any help appreciated. Thanks tranny1.