Most likely unrelated to your problem, but until 2 days ago I ran with auto-updated firmware and if I enabled multicast streams my router started rebooting every 3-6 mins interrupting internet..
Manually applying 112B19 firmware allowed me to turn multicast on without constant reboots.
ftp://ftp.dlink.se/Products/dir-products/dir-855/drivers_firmware/DIR855A2_FW112_B19.zip
If you're still experiencing the problem and if you havn't tried that firmware yet it could be worth a try, I guess.
Really wish D-Link would put some effort into customer support, I thought this forum was a great step in the right direction until I read this thread
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Employees ripping into disgruntled customers doesn't seem very professional, but then again that's just my opinion.