Geforce 680i LT Motherboard, 2x2048 Corsair DDR2, Intel Qcore6700 2.66ghz, XFX 8800GT vid card

I know I have seen this topic more than once, but I still haven't been able to figure out my issue. I started getting a CHECKSUM error when I would turn my computer on, and it would not boot past the BIOS. This only happened two times before it would not even boot BIOS, so I wasn't able to flash in time. I sent the mobo back and they sent me a new one. This did not fix the problem. Thinking it was the PSU I bought a new one. With the old PSU (550 watts), everything seemed to be running in the case even though it wasn't booting. With the new PSU (650 watts), the rear fan doesn't boot until I hold the power button in to force a shut down. At the moment of shutdown it clicks on for that split second. I have an Ultra PSU tester and when connected, all the lights light, except for the +3.3V 8-slotted port. I don't know why neither would light, especially for the new PSU. Also the motherboard, processor, and memory was bought together only 3 months before the problem.

I know it's a recurring topic, so any suggestions very much appreciated.