Originally Posted by silvia tech

It really just sounds like a ground that is loose or has corrosion build up. It will not always pop If your ground does not complete the circuit or lets say it is bolted down just not making good continuity due to corrosion It would also create this symptom. I don't think it is the cluster because I don't see that affecting the taillights at the same time if I were you I would Back probe the connector to the Multi Switch/Combination Switch (Light Switch)
Have a friend duplicate the symptom while measuring voltage and resistance you can also measure the continuity to see if it breaks up. Then if it is a wire follow it back until you find the problem. You might also find that the combination switch is internally separating, a very common problem on the 240's. If it is separating the switch might mechanically work but only give voltage to the correct terminals some of the time or might only interrupt voltage some of the time. I am not promising this is the fix and it is hard to say without being there. But that is where I would start.