2001 - lights flickering every now and then

Sometimes the lights inside flicker and at night, I notice the headlights flicker sometimes too. This mostly happened when we were doing the SeaFoam procedures and the engine was coughing for a second (as expected) but it still happens every now and then. I do have a stereo with an amp but it's a small amp at 200 watts so I don't think it should contribute to it that much and the battery is a awesome Optima Red Topper.
My mechanic best friend thinks it's a loose ground. Is that true and where should I look (under the hood or at the battery?)?