Intermittant Crash/Boot Fail

I have been having this problem for quite a while now and have been suggested many plausable solutions but each to no avail. Every now and then my system just seems to freeze and it doesnt seem to be during any specific activity and then when I reboot the system it comes up, I hear 1 beep and then it detects the hard drives, now it lists the two hard drives, but the second hard drive it lists always comes up garbled. After this the system just freezes. I can then reset the system a few times and it works again. I have tried memchecker and DocMemory and both say my RAM is fine which is what most people suggest is the problem. Sometimes though my system will work fine for weeks on end before this problem happens so I am completely dumbfounded as to what the problem is. Any help would be much appreciated.