Thanks to all who offered suggestions and advice and particularly to Don Wilhelm, who stuck with it through several rounds of off-list e-mails as I worked through the problem.
There may be something useful in the experience, so here’s what it was:
R7 is a ¼ watt resistor on the K2 front panel board, located about 2 mm from a grounded mounting screw. Although invisible to the naked eye, with a strong light and a magnifying lens that I used to examine every solder point on the board, I found a half inch curled up length of very fine wire, one end hooked on the R7 lead and the other swinging across the adjacent mounting screw, effectively but intermittently shorting out the IDAT line of the LCD driver.
It appeared to be a tiny piece of a single strand from the braid of one of the internal TMP cables that had fallen in during the build. It may have floated about harmlessly for over a year and then, with some odd movement of the rig, got one end caught on the resistor lead and the other brushing the ground point. Removed it with tweezers, reassembled the rig, and voila. All is good.
Note to personal checklist: Whenever possible, trim the TMP cable braids on the workbench, away from the rig, and brush off loose strands before installation.
Ted, KN1CBR
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