Slow multicasting

We are having problems multicasting images and we think this has started
since we upgraded from Zenworks 4 to Zenworks 7. We have a new Dell
Poweredge server for Zenworks, using 2 NICs for load balancing. The
image targets are HP D530s or Dell GX270s. Cisco network switches. All
multicasts are on the same LAN, so no routing. Have looked through
knowledge base and forums for something to try. We do multicast imaging
with Altiris LabExpert on another one of our LANs and it works fine.
Thanks in advance for suggestions.