Chosen Solution
My n64 plays some games just as they should, while some have a couple of levels that are too dark to play. Three games that come to find are 007 Goldeneye, Gauntlet (based off an arcade game), and Supersmash Brothers. A friend has copies of all three games that works fine on their n64 system, but my copies of these games on my console have darkness problems on a few levels, preventing the game from being playable.
Have you given your friend your copies to play and they’re also dark? Have you tried cleaning the N64 cartridge’s pins? Have you made sure the video cable is properly seated in the TV and console? Is the video cable getting interfered with by anything nearby? Is the video cable in a worn down condition?