Multicast issues in windows TCP/IP stack

Not sure if this the the right venue for this, but would anyone happen to know
if there is any changes or "enhancements" in the Windows 2008 64 Bit R2 TCP/IP
stack or in TOE that would cause multicast or microsecond bursts of UDP to be
dropped between the physical layer (NIC) and OS. I am running the same multicast
app on Windows 2008 64 bit NOT R2 or standard and do not have this issue.

-thx Philip