You have a/some devices wishing to join a group (the address 239.x.x.x will tell you which group). This is normal and is expected when an application wishes to join a group, it'll query it's local multicast router (switches too, will do this).
IGMP queries are used by the local client machine and the adjacent switches to connect the client(s) to the local multicast router. PIM is then used between the local and remote multicast routers to direct the multicast traffic from the source to the many destinations.
I'm not sure what you mean by an "IGMP flood". Do you mean all bandwidth saturated because of IGMP messages? If this is the case, how many multicast groups do you have? It MUST be a lot to fill a 1Gb LAN. Even a 100Mb LAN can handle a lot of multicast. Why are you using dense mode PIM? You do know that dense mode implicitly builds shortest-path trees by flooding multicast traffic domain wide, and then pruning back branches of the tree where no receivers are present?
Hope this helps.
