Some times when i switch the comp the mobo light comes on the fans start . But the system wont post . I need to remove the ram and clear the cmos and all and just wait abit for the comp to start . i remove the cmos battery and play hell for this thing to start . After doing all this it starts .
Check the cmos battery with a multimeter, sometimes when they die they short out and cause havoc with the bios, when the pc does post try running memtest to check the ram modules to make sure they're ok, try swaping out the psu with a spare if you can to see if it makes any difference.
It is your power supply, basically by doing all those steps you are resetting your power supply which is a switch mode power supply when you switch it off from wall switch you are resetting it and that's why the computer fires up