PC cannot boot, weird behavior

[Kiwi2703]

Hello there   
Yesterday I bought a new case (Corsair 450D) and new watercooling for CPU (Corsair H60). I installed all my components to the new case and turned it on, however, no signal was found on the monitor and the PC just turned itself off after a few seconds. Please note that the PC was working just fine before the case and cooling swap.
So I assembled everything on a piece of cardboard to prevent shorts, and connected only the bare essentials - the power, reset and LED thingys, the big motherboard power connector, the two 4-pin CPU power conectors and I even swapped the old CPU cooler (one of those Zalman copper thingys) and plugged it back in the CPU_FAN connector. So essentially, this is the same build as I had yesterday and which was working fine yesterday, just stripped off all the HDDs and USBs and other fans. I tried this with and without a GPU with the same results. I reset the jumper, battery, I even flashed the BIOS with a new version. Still the same thing.
I also noticed a weird pattern - when I leave it alone for a few minutes, then turn it on, it stays on for about 20-30 seconds then turns off. I cannot turn it on after that, first I have to unplug it, let the diode turn off, plug it back in, and then when I turn it on it starts only for about half a second. I have to wait another few minutes and then it starts for about said 20-30 seconds again. And this repeats.
I even measured the voltage on the PSU when not connected to the motherboard and everything was fine. I would like to measure the voltages on the motherboard as well but I don't know where to do that on my motherboard. It's ASUS P8Z77-M Pro. My PSU is Corsair TX650. My CPU is Intel Core i5 3570K.
If anyone could help me diagnose what's wrong with this system I would be really happy!! Thanks!