Thank you for your response. But no, the cooling fan was off -at least in most models, I only use it for those with fine details-, so it must have been something else.
Anyway, looking at the release notes (that weren't available when I posted, but thanks for offering) I don't see any overwhelming reason for migrating. I'll wait at least for 3.13.1... (a lesson I learned a long, long time ago -I started fumbling w/ PC's in the late 70's, on a Motorola EXORciser; Apple IIs didn't even exist yet- AVOID .0 software releases! )