First, sorry for 're-bumping' this thread, but I was just so glad. Just fifteen minutes ago I got news that mIRC v7.14 was out, so I (of course) downloaded and installed it immediately.
To my great delight, UPnP now works _flawlessly_. Bindings are suddenly working like a charm on both my D-Link DIR-615 and my D-Link DIR-635. I can only assume this would go for the well known DIR-655 as well, even though I haven't had a chance to try that one out. Like you said Khaled, it was probably something these routers requiring some extra up time for the connection to not get lost.
Anyways, just wanted to say thanks!
Good luck on further development of this brilliant IRC client!