Urgently need advice please

Hey guys I recently bought a bundle from tiger direct Erich included the following:
1. AMD FX8350 processor 4.0 GHz
2. Sabertooth 990 fx r2 motherboard
3. XFX RADEON HD 6870
4. Lsp ultra 550w psu

I post, it works great. Two days later I turn it on and it shuts off after4 seconds. Do it again, same thing. I return the psu and get a 650. Today, a day later, the same thing happened again. The motherboard light is on but every time I start it, it turns back off. Any chance it's the PSU or am I looking at a bigger problem? No beeps from the motherboard btw