you know I had the same problems yesterday - I found out thatWhat's happening is that the mobo isn't 'seeing' the HDD (and therefore not booting), but if I enter the BIOS settings, exit and then reboot, it sees it and boots fine.
the power cable to the harddisk stops working if I wiggle it
mine would do that - but then the computer would crash driving downbut if I enter the BIOS settings, exit and then reboot, it sees it and boots fine.
the road and come up to the bios screen , saying no operating system
found - I would have to reboot and re-detect the hard disk and it would
work fine - and do it again later at a random time - it was driving me crazy
I was going to start replacing hardware and Thought ,check
the easy stuff first
check the motherboard battery if it has one
check to see if there is a bios upgrade on the internet
try the bios defalut setings
try another harddisk on the problem motherboard
check the IDE and power cables