ECS PT800CE-A powers down in the middle of ...

What you are describing sounds to me that your BIOS on the MB is failing to POST. POST stands for Power On Self Test. The BIOS runs a set of rutines burned into itself or eprom that performs a series of tests before it goes to the HD to look for the OS. If the testing fails at any point the system will stop and the machine will power off. There beep codes associated with each of the major test such as the video card and the keyboard. Both of these devices must be attached and functioning to allow the MB to POST. If you hear beeps just before shutdown, try and listen and determine it code by telling if it gives a short beep followed by a long one etc... These codes can then be used to determine what is not passing the tests. Your MB factory or the BIOS Mfg will have a list with these codes to help you find out what's wrong. If you get no beeps, try reinstalling your memory or remove one stick and see if you can boot then. This is a common area of failure (memory) also the CPU itself could be the culprit. If you cant resolve by changing the memory orchecking out the CPU, I recommend you take the machine to a repair shop and see if they can run a POST analysis
on the machine before just dumping the MB. Also check ALL connections to the MB and be sure they are tight and secure and making contact. Hope this helps.