Kristen,
   
  Because the problem is so random, have you explored the possibility that perhaps the problem is occurring prior to processing?  We were having some problems with hard, shrunken tissue, and it turns out that the grosser was allowing the tissue to dry out before placing the cassette in formalin because it was "faster" :( .  This could also happen at the point of collection.  How does the tissue look under the microscope?  You could ask your pathologist if the artifact could possibly be air-drying artifact.
   
  Good luck,
   
  Shelly

       
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