Random Results on Export?

Hi all,
 
Every time I export this track, I get different results.  Everything starts out fine - the results are perfect (verified via null test) up until the automation curve brings up the wet level on an instance of BREVERB 2.  Then, things get weird - BREVERB's output is uneven, with random jumps in level and quality.  I've tried numerous exports but only a digital loopback provides predictable results.
 
Here's the track.  Not much going on but two instances of Izotope Ozone and the aforementioned reverb.  [Yeah, this is a mastering session.]
 

 
Here's the output on five different tries (magnified).  Everything is a perfect bit-for-bit match until the end, where the reverb comes in, and then you don't need a null test to see they're very different each time:
 

 
To clarify, the first is a 64-bit export to WAV.  The second is the same, but exported in real time - it's better but not perfect.  The third is a loopback, manually routing my Lynx card's digital output to digital input (24-bit) and appears to be ideal - looks and sounds great.  The fourth and fifth are identical 24-bit fast exports to WAV, one right after the other - they're different!
 
Any ideas what may be causing this?