Light is solid yellow on front switch, won...

if you are certain the power supply is good(try it in another machine) and its completely connected it sounds as if your motherboard is going bad, there is a 5-volt supply in your power supply that does not turn off when the system shuts down, its called 5-volt standby, this runs some small circuits on the motherboard that monitor the power switch, when its pressed the circuit starts up the whole power supply. thats how we can power off the system with software. usually its a custom chip and some components, but without the schematics of the motherboard and the replacement parts and the ability to de-solder and solder on a multi layer board its not usually repairable by the end user.

the system memory and processor are not even used in this part of the system startup, they come into play after the supply is up and stable and supplies the power good line to the motherboard.

make sure the supply is good, and all connections are making contact, that none of the pins have backed out of their housings when being plugged in and that there are no cracks in the solder joints where the power connectors attach to the motherboard.