Intermittent boot fault on A7V133 motherboard

Recently my PC seems to be freezing while in the initial part of its boot
sequence - before it wakes up the monitor.

I press the "ON" button, the PC starts up: but the led on the monitor
remains amber. Although I see the LEDs on my CD drives flash a couple of
times I don't hear the floppy being tested for a disk in it. After a while
the HD turns off and my computer just remains there, switched on but with
the monitor in standby.

I thought at first the problem may have been something to do with my
graphics card - though I'm not getting any Beeps Error codes on start-up. So
I've replaced the graphics card with a brand new one and the problem
persists - sometimes pressing the reset button on the computer will make it
boot up normally; waking up the monitor and everything running as it should;
other times the monitor simply refuses to wake up.

I also tried another monitor to see if that was the source of the problem -
but it made no difference; as does switching the monitor on and off.

Anyone have any idea what the problem could be and how to fix it?