You'll have to contact your webhost either via a support ticket, or in the customer forums, and ask if they run PHPSuExec so that PHP processes are run as the owner. They are probably not though as that is the exact problem that happens when PHPSuExec isn't being used. Example: HostGator have it, and I have all my directories set to 755 yet Joomla says all my directories are writeable
I think you might actually need media as well as cache and tmp to be 777 anyway....? As far as I know, the media folder is used for people to 'upload' their images/attatchments for many Joomla extensions such as when writing an article in the JCE Editor - I can't remember exactly.
Under media/ though, there should be a folder named myjosctemplates - if not, create it. Then try setting only that folder to 775 and media back to 755, see if it's any different. That folder is used for the 'Copy Template' function of the backend from memory.
Regardless, this is probably a bug in JoomlaComment that could be fixed (trying to write there even when it doesn't need to). It shouldn't need to write to the media/ folder at all from what I can see.
EDIT: It's on the forums of my webhost, a lot of it may be irrelevant but it is a great "WTF" on what PHPSuExec is. You can see this public announcement of when they installed PHPSuExec on their servers at
forums.hostgator.com/phpsuexec-being-ins...vers-tips-t8822.html
- you will see that they also reference the Joomla CMS in the announcement.