6 year old Shuttle PC starts up ONLY every second time... stops just after Windows logo and 'rotating spots' view!

Has been suggested that I have a 'crack' in motherboard or associated wiring which closes up soon after heat associated with power source.

Hence no power to hard disk.

Any other suggestions would be welcome!

Typically what your describing is the point at which windows starts to load it's drivers before booting to the desktop. Often the graphics drivers are the cause but any driver could be the issue. A 6yo. PC might have a low motherboard battery that has enough juice to keep the date and time but is failing to keep the bios correctly so a second try might have reset the bios enough for the PC to boot correctly on the second try. Then after some down time it happens again. If you have a multi-meter you can check the button battery to see if it's reading 3volts. If it's reading less replace it. Then if that isn't the case I would reinstall my drivers over themselves (the same version) to get rid of any possible corruption starting with graphics. Corruption of drivers can occur from a bad shut down or a storm outage. You might also consider looking in Device Manager for any issues that show up there. A cracked motherboard is unlikely unless you dropped the darn thing. Sometimes re-seating your memory will solve the problem as well.

Are you getting a blue screen of death or is it just going through the boot cycle an extra time ?

Im asking because that also can be part of being able to diagnose what is going on also you havent said if it does run after the 2nd boot process. Is this unit windows XP ?