Well, as it turns out, I now know a lot more about seat geometry and construction.
There are 4 bolts that hold the seat pan onto the clamp that slides up and down the boom. The clamp is actually two half-clamps that join at the top and the bottom. When you tighten the quick-release on the seat, you're actually closing the bottom of the clamp. The top is attached to the seat pan.
I had lost two of the bolts, and one had broken. The result was that half of the clamp was free on the top, so the thing wouldn't close around the boom. This allowed the seat to seek its own position, irrespective of my wishes.
Now that the mechanic actually believes I ride as much as I said I do (why don't they *ever* believe you??), I've got 4 new heavy-duty bolts, a stronger "no stretch" chain (the old one was in dire need of replacement), some new brakes (the old were on their way out as well), and new heavy-duty cable and housing for the rear derailer. When I get enough cash, I'll replace the wheelsets and brakes with a switch to disc/
At least now he understands that I'm serious when I tell him I'm putting 150-200 miles/week on the bike under all riding conditions.