If the car is otherwise running great, my guess is that it is a gummed up O2 sensor. Even if it was replaced, they sometimes go bad. Basically, you could check all the wiritng to it adn if that all looks good, you're back to an O2 sensor that is occasionally slow to respond, thus throwing the code. If it were me, I'd probably just figure on replacing that oxygen sensor.

It's a bummer that they're so expensive though...

hope this helps,

Nate
--
1997 850, 1979 242GT, 1990 740GL