Complex sentence: In 1990 he was visiting Professor at Stanford University in 1990, and then a lecturer at the London School of Economics, becoming a professor in 1990 and head of the Economics Department in 1999.
Split into simpler sentences: In 1990 he was visiting Professor at Stanford University in 1990. From there he became a lecturer at the London School of Economics, becoming a professor in 1990 and head of the Economics Department in 1999.