Suspect the generator. Unplug the small connector on the side at the back of it and see if the flickering stops. When one of the six diodes fails, "ripple" will be very high and the unit will only be able to produce exactly one third of its rated output current. That's not enough to meet the demands of the electrical system under all conditions. This is a real common problem on GM vehicles and you can expect to go through four to six generators in the life of the car. To reduce the number of repeat failures, replace the battery at the same time unless it is less than about two years old. Due to their design, these generators produce huge voltage spikes that can destroy the internal diodes and voltage regulator, and interfere with the many computers on the car. As the battery ages it loses its ability to dampen and absorb those spikes.