thanks b.c., jp and art.
the car sat in the garage all weekend while i mowed the lawn and thought about the problem. this morning i had to drive #2 daughter to the bus. the idiot light stayed on all the way 'til we turned onto the hwy (1/4 mile), then went out as the engine speeded up. further checking is very consistent: at start-up the light stays on while idling, but will go out when the engine gets up to about 200 rpm (no tach, just guessing), then it goes out. it stays out even with electrical load and back down to idle -- in short, it functions normally once the engine first gets up to speed. time is not a factor -- the light will stay on as long as it takes to rev the engine -- 5 or 10 minutes, even; but it will go out right away with a quick "pop" of the throttle.
all this is confirmed with volt meter as i go through the drills.

now, on to the tail lights...
dave