To make a long story short, I pulled the drill apart. The knob that adjusts the clutch has a little tab that hits a stop.
A small pair of side cutting pliers works nicely to remove the stop. Now the clutch adjustor goes farther to the tighten direction. Seems to clutch a bit better, now.
Amazing how you can break things and make them work better. Wonder how much longer I'll get out of this drill. Can't remember how old it is, but probably two years of good service on it.