from where an error that can not easily be duplicated and seems to occur randomly?

Answer 1

i hate to answer a question with a question, but can you elaborate and ask again?

Answer 2

It could be many things:
1. Defective RAM (or not enough voltage - is your RAM overclocked?)
Test it with a special utiliy like memtest86+ (requires you to make a boot floppy or CD). But before this, enter bios and load defaults.
2. Overheating of the CPU or of the mainboard's chipset (are your CPU and cooler properly mounted? is your case properly ventilated?)
2. Unstable voltages from the power supply - you can easily test this using a voltmeter. There are different voltages feeding the various components such as hard drives, mainboard and the others.
3. Unstable voltage control by the mainboard (for example not enough voltage to the AGP interface or to the CPU).
The temperatures of the CPU (and maybe others too) and some of the voltages can be monitored within windows using utility software such us MBM5 or speedfan (depending on your motherboard's manufacturer, you might have a compatible program on the supplied CD, such as Asus PC Probe or Chaintech DigiDoc).
Good luck!

Answer 3

Use standard fault finding techniques. Take an 'image' of the hard disk and try to reproduce the fault on a second (test) PC.,

Remember - time is money - if the problem is restricted to one PC and can not be reproduced on a second PC, swap the PC's over and scrap the one that failed (don't forget to remove and crush the Hard disk first).