I'm having the EXACT same problems.
I get frequent dropouts even though the signal strength is 75%. The signal strength doesn't surprise me, given that I'm upstairs and my DGL-4500 is downstairs. But every time it reconnects it shows reasonable signal strength and sometimes just immediately disconnects again, which doesn't make a whole lot of sense. There are plenty of other 2.4ghz networks, but no 5ghz networks, which is where I'm operating.
Heavy use will apparently crash the Vista USB functionality and lockup the network functionality. The network icon shows I'm still connected, but it's dead -- hovering over it doesn't make a tooltip appear, double-clicking on it does nothing. Bringing up the network properties just produces a blank box. The adapter itself keeps blinking as if there's no problem. If I unplug the device, nothing changes. I can't plug other USB devices into that port, either. I end up having to power-off my computer (it won't shut down). So, it appears that heavy use causes the Vista USB support for that port to crash.