Try these.... if you haven`t already:
*Cookie Folder Location
This issue can also occur if your Cookies folder is located on an NTFS partition and you do not have at least Change permissions for the Cookies folder. To resolve, make sure your account has sufficient permissions on the Cookies folder.
*Incomplete Page Download
If you press the ESC key before the page has completed loading then the "View Source" menu item may not be displayed.
*View Source Restriction
The View Source function may have been restricted by your Administrator. To resolve, see the "NoViewSource" registry value in Internet Explorer Restrictions.
*Invalid Temporary Directory
You may also receive this error if the path specified for the TMP environment variable is invalid. To resolve, make sure that the TEMP directory is a valid directory and that there is sufficient space on the drive.
*Invalid View Source Program
If you have specified an alternate program to view the source code. To resolve, make sure that the alternate viewer is valid and operational.
*Right-click disabled by Web Page
Some web pages may implement a restriction to stop visitors from right-clicking on the web page. The source may still be viewed by clicking the main "View" menu, then "Source".