Tach up and down on idle
I've noticed that when I'm sitting at a red light, my tach will be up and down. It dips way down, the engine sounds like it's going to stall, and the lights all dim. It only started doing this recently. The idle usually sits around 1000, dips down below 500, then goes back to 1000, might go up to 1500, then back to 1000, all of this while I'm not even in gear. What could be making it do that?