[idvusers] How do you time step 3-hourly NARR data in the IDV?

IDV Users,

I'm not sure how many of you interrogate NARR datasets on the IDV, but I'm
doing more of it now through the IDV using bundles and ISL scripts.  One
big limitation I am finding is that I cannot time step/animate through more
than one data file on the Map View pane.  For instance, if you load in
fields from an 18z file and then also a 21z file, the IDV just displays the
21z data right on top of the 18z data.  How do you configure the IDV such
that for these hourly dataset files you can create animations and step back
and forth through various times?  Any help/guidance is appreciated!  Thanks,

Mike U