Took her out for a spin yesterday.
Initially, the left EGT gauge was a little intermittent, so we did a high speed taxi and returned to clean the cannon plug in the firewall and re-safety wire that. On second startup it seemed a lot better, so we decided to go for a spin, which was fun. I had instructor with me, as it had been awhile since I was up in the air. Made a 1hr loop to shake things out, but the EGT was still shorting out and dropping intermittently. Hmm, so the cannon plug cleaning wasn't the solution...
And after calling my old friend Stan Perkins, who's a guru with his 681, I now understand why: The engine plug in the firewall isn't the cannon plug at all. It's the banana-plug next to it. We'd been tightening and cleaning the wrong plug!
So next week we'll clean and re-tighten that and hopefully that will take care of it. If it's still intermittent after that, only thing left to check is swapping the EGT gauges top see if follows the instrument itself.
But we're close. Real close to getting out of there.