You guys have short memories. The Chargers weren't very good in the '80s (kinda like the Raiders are now), but the rivalry was red hot while both were very good teams in the late '70s. That's the problem, it's rare that both those teams were good at the same time.
They were good for at least one year in the 80's. In 1981 when they played the Bengals in the AFC Championship game which was the coldest, if you factor in wind chill, game to ever be played.