Windows 7 Boots on Restart, but Fails to Boot After Shutdown

Hi,
I have been having this problem for the past 2 months now where my computer would not start after complete shutdown, but if i start the computer press F8 and boot in safe mode it boots ok and if i restart it, the computer starts back fine. I have tried diagnostic tests on my hardware (HDD, RAM, CPU, MB) and all have passed. Neither has there been any voltage fluctuations or any type of physical damage to my computer hardware. What could be the possible cause ?

Hello and welcome to the forum.
A very difficult problem to diagnose.
My first suggestion would be to examine the startup tab (msconfig) and see what you have running at startup as well as the services tab (hide all microsoft services) once you are familiar with what's what there consider using How to troubleshoot a problem by performing a clean boot in Windows Vista or in Windows 7 to see if you can identify the offending program, process, service or combination that may be causing the problem.
Other than that, it could be anything from a cold solder joint to copper tracings on the motherboard that require a minor temperature increase in order to expand sufficiently to make and complete the circuit. I would guess that having already performed the hardware diagnostics that you've indicated that you've already inspected the individual components and made sure that all cable connections (power and interface) are seated properly and solid as well as all add-in cards and modules are likewise seated firmly and completely.
Regards
Randy

Hello,
Thanks for the response. I think i might have fixed the problem. I removed the PSU from the computer cabinet and Its main power connection to the motherboard and reconnected it again, so far for 2 days its been acting fine and booting up properly.

Glad to hear that you were able to resolve your problem. Thanks for posting back and updating your thread with the solution that worked for you.
Thanks for joining our community and we hope to continue to see you around the forum.
Regards
Randy