I think you have a few things mixed up. As I understand it, DAB is still radio wave based, like FM, but it is encoded in such a way that it is a digital signal, not analogue. I.e. DAB is not the same as an internet stream. Unfortunately, the Desire HD only has an FM receiver, not DAB.

However, as far as I'm aware, Tune-in is internet streams only and does not make use of the FM receiver. So if you're getting a flaky signal, I'm afraid that's your 3G conntection and streaming it from their site won't make a difference. (It probably is a plugin issue though - it appears to be a flash based player, so it wouldn't surprise me that it's temperamental on our phones).

What you could try is going to Tune-in's settings and increasing the buffer length from the default of 5 seconds to the max of 30 seconds. There will be a longer delay when you first hit play, but that would make drops in signal less noticeable, so long as they weren't too long