Chocovision Tempering Question

Forgive me for the double post--I "commented on another thread, and want to post this as a new discussion too. Thanks!

I've been running 3-5 Rev Deltas pretty much all day every day for 4 years. Recently, I've had a problem that I haven't been able to figure out. A machine will near the end of the tempering process (in the period after "seed out") and will over crystallize and begin to bunch up against the scraper at the left side of the bowl. Ambient temperature is the same for all machines (~73 degrees), relative humidity <45%, no water dripping/seizing of the chocolate (the same chocolate can be retempered in a different machine later with good results). I've moved the machines to different locations (to control for drafts, etc.), tried different baffles, sent the beasts back to Chocovision (they find nothing wrong). It only seems to happen with my 68% dark chocolate (not white or milk). Has any one experienced this and found a way to address it? Thanks for any help.