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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 (less than or similar to1 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 similar to40% peak to peak on timescales of several months. Its low Galactic latitude of b approximate to - 10degrees 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 approximate to +69degrees 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 less than or similar to 1 yr time baseline.