Started out with a multi-factor problem, so I started from the top. My main point of contention
Fan clutch had gone out and radiator had cracked so I started there and got those two things replaced. That at least got my AC system working again (due to fan clutch, right? Before, it was clicking but would never blow cold air.).
But once it started blowing cold again, it was only out of the defrost vents, but maybe the dash at sparse, random intervals. Such as a few minutes after startup and then once every 5-10 minutes for a minute or so at a time. Researching this, I come to find I have no vacuum controlling the dash air mixers. So I traced the lines...
My black vacuum line had some heat stress and age degradation just off the vacuum reservoir headed towards the firewall. Clipped out the gnarly bits and stuck both clean-cut ends in an oversized splice-connector (the old hard brittle plastic line is too small to use barbed connectors IMHO), and used electrical tape super-snugly to hold the hose inside the splice piece. VOILA. Dash control diverts air properly again and I didn't see the AC drop out...yet (haven't gone out for more than 5 minutes but I wanted to at least share that identifying and repairing this vacuum leak did get dash air mix control back). At least if the AC does drop out again I know it's probably just a relay or pressure switch issue since my system works well when it is active.