That depends, is the multicast routing only between vlans? Dense mode is a push and prune where multicast stream is pushed and the routers basically send prune by those who does not want the mcast. Dense mode basically is the opposite, where routers running multicast have to join and that's how the stream gets forwarded, this is a nutshell, of course.
What you need to get is show proc cpu when the cpu is high, to make see which process is causing the cpu to shoot up. Next, make sure that you have igmp snooping enabled on all the switches, who knows, it might be getting flooded. More information is needed
