Computer repeatedly turns on and off.

All right, I am sick of working on this thing and I really need some help.

I've been trying to build a PC. I have all the components and I triple-checked that they were all connected properly. However, upon trying to boot it up, it doesn't even hit POST. The lights turn on and fans spin for about 4 seconds, then shut off, then power up again, and repeat.

I just did what I think is my final test for this by turning it on with the motherboard outside of the case, and the same thing happens.

I'm completely stuck here.