Newly built computer turns on and turns off after ~5 secs!

Hi
I just built an Intel core i5-4670k, 8 gb Corsair Vengeance ram, ASrock Z87 Pro3, and Coolermax ZU 600b all in a Thermaltake Comander MS-I Snow White. Its my first build but I have experience when it comes to computers.
I built it a few days ago, and it worked flawlessly (I have no OS on it yet, so I just checked out the BIOS and turned it off). However, as of a yesterday, when I tried to turn it on, it simply turned on then off immediately. The fans, hard drive, computer, and everything else stayed on for a second, and then it turned off, then back on and so on...
I read some forums with the same problem, and yes, I know about the checklist http://www.tomshardware.com/forum/261145-31-perform-ste.... I removed the 8 pin and 24 pin connectors and that seemed to help. Now, when I press the power button, the computer starts up (so this can't be a faulty power button) but when I try to turn it off, it stays on. The computer stays on for about 5-7 seconds and then turns off for about 2-3 seconds and then turns it back on.
I am currently "breadboarding" the rig (take everything out of the case and build it externally). However, PLEASE help me! This is my first ever build, and I'm scared as hell on what to do! Thanks a lot!

Also, just as a sidenote, when I was doing cable management, I accidentally pulled a bit too hard on the USB 3.0 connection from the case to the mobo. About 6 pins broke. As I tried to fix them, I bent more. After reading, I learned that there was nothing I could do about it. Its alright, its only one USB port in the front of the case. However, can this be the source to the problem?

Also another thing. When I was first installing the aftermarket heartsink, I was having problems. When I was repositioning the heatsink, some thermal paste got on the mobo. It wasn't bad, just a string, and I cleaned it off immediately. I ended up opting for the stock fan because the aftermarket heatsink wasn't fitting in the mobo. Gonna return it. However, the computer worked perfectly after that, so I don't think that's the problem.

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Solution!: After breadboarding the rig, it started to work perfectly fine! I guess something was up with the wiring.

If you ever get in this kinda trouble, just take the build out of the case and try building it without the chassis. Also, some people I talked to told me to recheck the heatsink, that is most probably the number one cause after wiring.

Thanks for the answers guys!