[fixit-p]

You said you checked for voltage at all the lights and you have good readings of +12 volts, I assume you did this when the fault was present. If that's the case the problem is most definitely the ground side or open circuit to ground. You also said you checked for continuity to ground but that test can be misleading because any path to ground will give you a good reading. Next time you have an issue check for voltage at the hot side of the load then check for voltage on the ground side of the load (the taillight housing etc.). If you have voltage present on the ground side it is because the ground is poor or missing (anything over .5 volts on the ground side is an indicator of a poor ground). If you have 0 voltage on the ground side it means you have an open circuit to ground at the load (could be a bad bulb or bad connection from the bulbs shell to the taillight housings socket). These voltage/voltage drop tests will pin point your problem, you can post back the results for interpretation.