The nucleus of the Seyfert 1 galaxy NGC 4151 undergoes large-amplitude variations and occasionally passes through deep minimum states lasting up to a few months. Two narrow emission lines, L(1) and L(2), variable in intensity on a time-scale of a few days were discovered during International Ultra-violet Explorer (IUE) campaigns at low states in 1981 and 1984 and were identified with components of C IV lambda 1550 blueshifted and redshifted by -6100 km s(-1) and +8500 km s(-1) respectively. We re-analyse in an homogeneous fashion these data and additional unpublished data obtained with IUE in 1986 and 1987. This analysis confirms the previous results and adds to our knowledge on the longer term behaviour of L(1) and L(2). Specifically, (i) at each low state, the two lines re-appear with approximately the same intensity range and variation time-scale as in previous low states, even though in the meantime the nucleus passed through bright episodes with strong continuum and intense broad emission components. (ii) There is no detectable wavelength drift of L(1) and L(2) in 6.7 yr. The observed wavelength of L(1) remains constant at 1522.75 +/- 0.4 Angstrom. (iii) There are correlations between the intensity of L(1) and the intensity of L(2), and between the intensities of the two lines with the continuum flux. These correlations are strongest in 1981 and 1984. We briefly discuss various interpretations and conclude that, at present, none is satisfactory.