We present flux ratio curves of the fold and cusp (i.e., close multiple) images of six Jodrell Bank VLA Astrometric Survey and Cosmic Lens All-Sky Survey (JVAS/CLASS) gravitational lens systems. The data were obtained over a period of 8.5 months in 2001 with the Multi Element Radio-Linked Interferometer Network (MERLIN) at 5 GHz with 50 mas resolution, as part of a MERLIN Key Project. Even though the time delays between the fold and cusp images are small (<~1 day) compared to the timescale of intrinsic source variability, all six lens systems show evidence that suggests the presence of extrinsic variability. In particular, the cusp images of B2045+265-regarded as the strongest case of the violation of the cusp relation (i.e., the sum of the magnifications of the three cusp images add to zero)-show extrinsic variations in their flux ratios up to ~40% peak to peak on timescales of several months. Its low Galactic latitude of b~-10deg and a line of sight toward the Cygnus superbubble region suggest that Galactic scintillation is the most likely cause. The cusp images of B1422+231 at b~+69deg do not show strong extrinsic variability. Galactic scintillation can therefore cause significant scatter in the cusp and fold relations of some radio lens systems (up to 10% rms), even though these relations remain violated when averaged over a <~1 yr time baseline.