Please help! Screen goes black.

Hi, please help, my computer doesn't boot, the screen just go black! * The system starts, all fans PSU, CPU, graphics card fan, starts, * The HD sounds like it's running * I hear no beep from the mother board when it starts. * It's possible to shut down the system by holding the power button. It worked fine until one day when trying to start it the above was the case. My friend has an exakt copy of the system, all tests have been done between our machines. This is (a sample) of what I've tried: 1. Exchange graphics card to a working one. 2. Test RAM in another machine 3. Test CPU in another machine 4. Bought a new motherboard and replaced the old one. 5. Changed battery 6. Tried the system without HD, Keyboard and mouse connected (in all variations) 7. Checked all chords (also pulled them out and puttning them back when changing mother board obviously) The strangest thing though: After replacing the motherboard, the computer started ! (with a minor delay) Then it worked for a few minutes, started up windows and everything. While we were toasting with a fine whisky it died again and all was back to "normal", i.e. the problem persits! I have no idea what to make of this! Please help, I'm pulling my hair out! Many thanks in advance, Johan ***Specs*** Antec Chassi 450W Sonata II - Piano Black Gigabyte - Socket 775 - ATX IP965 (GA-965P-S3) - DDR2 / SATA2 / Gblan Intel Core 2 Duo E4300 "Allendale" 2x1.8GHz / 800MHz / 2MB (Boxed) Kingston DDR2 2048MB 667MHz CL5 (Kit of 2) Samsung 250GB SATA II Spin P120 NCQ (8MB Cache / Sata) Samsung Intern SATA DVD±RW 18x - Svart (Dual Layer / SATA) (Bulk) Asus ATI Radeon X1950 Pro 256MB DDR3 (EAX1950PRO/HTDP/256M) - PCI-E / Dvi