Pc wont post

Hello all, So the last few days that i've turned my pc on it has come on, then after a few seconds it cuts off and then when i turn it back on it fires up and runs. it's done that for about 3 days. also i noticed it has been veryyy slow on shut down..Today the computer come on, stays on but my screen is blank. "yellow" light is on the monitor. all fan's are turning and lights on, can also hear the hard drive clicking and the cd drive still opens and shuts. also the keyboard and mouse don't light up. I've done some research and can't seem to figureout what the problem is. Things i've done so far :
1.First i cut it off and made sure there is no lose connection anywhere, everthing is pluged in tight.
2.i went on to unplug all my USB's and IDE things form the pc and still nothing.
3.I have 2 4870 video cards so i took them both out and put one in at a time trying each slot with both card and again nothing.
4.also checked the ram, took each stick out and checked them one by one to see if the pc would start and again a fail.
5.I put a new power coard i had on the computer and also took it to another room with a diff monitor and tryed that. unpluged the harddrive and pluged in a older IDE one i had.
6.Looked over the mobo to make sure i didnt see anything that looked to be burnt etc all checked out.
7.Reset the jumper hopeing that would help, fail.
So my question is what else can i try? is there a solid way i can find out if it's the motherboard? or maby it's the proceser? any input would be great

some more info.

Asus P5q Mobo , Gskill 1066ram-q9550 quad core processer -1tb Wd sata hd. 2 MIS 4870's. 900wat psu.