Multicast... Wireless... Why?!

Set up a multicast server yesterday (on my newly acquired DL380 G2, thanks Mark :)) relaying a simple 2 channel audio stream from the Internet edge to the LAN.

The problem arises with the wireless access points - the stream is skippy and rubbish when using the wireless connection - and the access points are going absolutely nuts.

Trying to troubleshoot the connection, I've made sure the server isn't sending out a rediculous amount of data. It's not, it's sitting around 0.2mb/s with no regular spikes in traffic. MTU should be regular ethernet. Checking connection usage on the wireless client (receiving workstation), it's also sitting around 0.2mbps, so that makes sense, it's receiving what the server is giving out.
But the access point is still going absolutely nuts, and in some cases traffic through the wireless connection slows to a crawl.

First thing I can think of is that the packet rate is way high, meaning that overhead associated with each packet is saturating the link, but this would show on a capture, and also show as network utilization.

Next thing is that maybe it's not to do with the IP layer, but the wireless transport - i.e. beacons or some such thing. This would explain how we can saturate the wireless link but not have it show up on the network utilization monitor. But why?

This doesn't happen with unicast streaming, which I have successfully cast over 7mbps MPEG2 video over wireless LAN using HTTP, which would create a wide range of packet sizes and retransmissions, surely enough to test for a 128k MP3 stream. So it most likely is multicast causing the problem...
Is the MTU set too high causing fragmentation?

Is it a propagation process that I don't know is happening? There's two wireless points on the network, could it be that? The server is using LAN teaming, but the older server I tried this on did not have that so that can be ruled out?