System will power down, but not switch off ?

When "start,turn off computer,turn off" is used to turn off the pc the screen go,s off the pc seems to get to the power off stage and then reboots or re-starts but not fully ie the screen will not appear ?
You just here the fans going. If i hold the power button in for 6-8 seconds the system shuts down, and then starts ok when the power button is pressed
Any ideas ?

Thanks in advance

Trevor S