One issue which may be happening is a problem with the alternator.
To explain, I'll give a brief outline of how an alternator works. The alternator, as the name implies, creates alternating current (AC), then through a set of diodes, rectifies (I believe is the correct term) the electricity to create direct current (DC) which the vehicle can use. The circuitry looks something like the following:
In the center of the image, where you have the three curly cues, are the field terminals where the power comes out of and power is forced through the diodes which are to the right of it. Each field terminal can only "push" electricity through the diodes in the direction of the arrows (representing the diodes). During the alternating process of electricity's creation is a wave process, it only pushes electricity through as DC current during 1/2 of the wave cycle. The reason there is three field terminals is so the three "output" portions of the current being created covers the greater period of time while the alternator is running. If you only had one field terminal and one diode set, you'd have fluctuating energy creation which you'd be able to see in the lights. If one of the diode sets quits working (gets burned out or whatever), the alternator would only be producing electricity during 2/3 the time it would normally. When the engine is slowed down during idle, this dip in energy production becomes noticeable and why you might be seeing the fluctuation in light output from the tail lights as you're sitting still. As the engine speeds up (not at idle anymore), the time period between energy creation gaps becomes shorter and it is much less noticeable.