particle render changes position

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I've got a particle sim - four particles with geometry copied on.

Sometimes particles die and when they do a new one is created,

and I'm using keyboard input to sync up some other actions.

I've found that if I step through the movie with the plus key (and do my keyboard entries), when I subsequently render the whole thing, particle positions go out of sync about half way through, but if I play it back in wireframe everything is ok

I haven't got anything being copied to the particles at render time, I dont have the real-time button on - could anything else be causing this?

thanks mark