why does my cluster form but fail after a few minutes, or why do my multicast communications stop working after a short amount of time?
Because you have hardware problems?
Because you have software problems?
Because you configured your cluster incorrectly?
Because your network is under attack?
Because your network is configured incorrectly?
With all of the data you have given us to diagnose your problem, it is hard to make any guess (even uninformed) about what your problem(s) might be.