afni.nimh.nih.gov

You could use a command like

3dmerge -1clust_order 10 422 -1thresh 1.965 -1dindex 4 -1tindex 5 -prefix clustered_dataset dataset+orig

to create a dataset where the i-th cluster has the value i stored in its voxels (i=1 for the largest cluster, i=2 for the next largest, et cetera). This may help you figure out what is going on.