If the cartridge is suspect, then the first thing to do would be to test it with an OHM
meter (multimeter). One probe to "R" (red) the other to "RG" (green), then do the same for "L" (white) and "LG" (blue). The readings should be relatively close.
If the cartridge is o.k., then the next step would be to inspect the stylus close-up. The cantilever should be centered without kinks and the tip should be free of gunk build-up. Click on the link below to get an idea of what the tip should look like magnified.viewtopic.php?f=19&t=11431&start=975