This can also be fixed by using an 0 Phillips screwdriver to adjust variable resistors VR1 or VR2, accessible by removing the battery compartment cover. VR1 and VR2 are located either side of the fastening nut for the cover.
VR1 (left of the nut and closest to the GBA game slot) adjusts the upper LCD and VR2 the lower LCD.
Slowly and gently turn the screw until the shimmer is eliminated.