Nonuniform lines on finite projective planes

We consider the stability version of the following problem, originally posed by Erdős: colour the points of a projective plane of order q, q odd, with two colours. What is the minimum number of nonuniform lines, that is the lines on which the number of points of the two colours are not the same. It is easy to show that the number of nonuniform lines is at least q+1 and there is a trivial colouring with q+1 nonuniform lines. Our main result is that the number of nonuniform lines is at least 13/8 * q or we have the trivial colouring.