Michael - sorry I don't know a whole lot about media share, I don't use it myself.
In my experience wireless is pretty stable unless there is something else interfering with that channel. Unless you're having problems with other things, it should not be that. You are using an access point hooked to the ethernet port, right?
You could try giving the HR21 a static IP instead of using DHCP, not sure if that may help?
Of course, you could always run a wire to it temporarily and see if the problem continues.