strange error from AD when using powershell

hey guys, im trying to add UPN suffixes to AD-Forest, and im using the below command for that.

Import-Module activedirectory
Set-ADForest -Identity domain.com -UPNSuffixes @{Add="demo23.com"}

now the problem  sometimes im getting the below error.

Set-ADForest : Insufficient access rights to perform the operation
At line:1 char:13+ Set-ADForest <<<<  -Identity "domain.com" -UPNSuffixes @{Add="demo23.com"}+ CategoryInfo          : NotSpecified: (Microsoft.Activ...gement.ADForest:ADForest) [Set-ADForest], ADException+ FullyQualifiedErrorId : Insufficient access rights to perform the operation,Microsoft.ActiveDirectory.Management
   .Commands.SetADForest

Now, the above error comes occasionally.. 

i was able to actually execute the powershell cmdlets peacefully, then all of a sudden it throws teh above error for teh same user on same machine, and on same powershell console.. 

im totally blank here. can any help help me with this strange error.

Edit:

Just now i have been able to reproduce the error. What i did was to log off from the domain controller and then after logging back and running the same command, im getting the same error.