Can anyone help with this problem, it's driving me nuts!

My PDC, at any random time of day or night decides to stop functioning with it's major administration tools,

For example when you open :

active directory - server is not operational
group policy - server is not operational
event viewer - access is denied

however my exchange server active directory allow me to load it up, reset passwords etc, modify the domain group policy and works as per usual.

The only way to sort out the problem in to reboot the PDC server then all seems fine agin, until it randomly happens again

Can anyone point me in the right direction as to why this is happening?