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Delete SOP making particles go crazy


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I've got a simple particle setup, a large number of particles being advected by a smoke sim and then cached out to a bgeo.  Then loading that into a File sop and using a delete SOP with a Delete by Range (29 of 30) to dramatically thin it out for some close ups.  This process has been working well for me across a variety of different particle setups, but for some reason on this shot, my particles hit a certain frame and go bonkers.  There is nothing wrong with the actual sim, so I can only put it down to being something to do with the delete SOP.  


Any ideas what this might be causing this?  

And any way to fix it without changing my existing geo, as I had already rendered hundreds of frames before I noticed this problem and would love to not have to redo the earlier frames, if possible. 

Thanks for any advice

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What Skybar said, what is happening here is the pointnumber of each particle is changing on every frame so if you are deleting by that a particle will be deleted on one frame and not another. 

If you use id instead you will get the result you want as id stays the same over the frame range.

The deleting by ptnum might have been working before if you were not sourcing new particles or deleting old ones and the ptcount stayed the same, what is probably happening on the frame where it goes wonky is the point count changes.

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