In answer to some questions.
 Yes, we have just had a software upgrade. 
 Yes we have had tech service in to check things out. (We are assuming that
the vortex mixes were looked at in addition to other instrument maintenance.)
Our first thought was a dispenser issue, (we are using Ventana dispensers)
but we have used several different dispensers and lot numbers.

  We are baffled ! Keep those thoughts coming. We appreciate your input.

Lynn