Are these temperatures you mention ambient room temperature? Or temperature that you've measured inside the cabinet somehow?
If those temperatures are "as measured inside the cabinet", failing above 72 degrees is a pretty low "high" temperature. I would expect maybe something up around 90 or 100 degrees F. And failing at LOW temperatures, specially when "low" is 67 degrees (F, I presume) is *HIGHLY* unusual. Electronic circuits usually work better at lower temperatures (to a point).
If those temperatures are room ambient, have you measured the temp inside the cabinet? And specifically near the PMDX-120?
Regardless, if you don't feel up to trying to swap the IC, as I mentioned before, contact PMDX and see about an RMA. They may be able to take a look at it and see if there is some other issue, or they may be able to swap the IC for you. Cou;dn't hurt to ask.