Compaq Presario Power Up Issues

Hi - I have a Compaq Presario (4 years old-ish) that has had a strange problem for about 4 weeks... When I switch on I get the power light, the fan noise and [sometimes] the caps lock light. However, approx 2 times out of 3 nothing else happens! I then have to hold the power switch for 5s to shut down, and then try switching on again. It can take anything from 2 attempts to 15 attempts but always turns on in the end! I know it will switch on when I see the caps lock light flash slowly. Once on, it all works fine.

I have tried with battery in and battery out - same result. I have tried a different power supply. I have tried different RAM. I have tried holding the power button down for 60s. No change!

Is this just a sign of a dying motherboard, or is something else going on here?!

Any ideas much appreciated!