I would check your fuses, also. I had to replace that very fuse before it would act right.
Everything was fine until I decided to do some maintenance and clean the contacts on the fuses.
Then I sandpapered that fuse and used dialectric grease, but it just would not give me consistancy.
One day on a trip I found myself in a dealership, so I bought a couple of fuses. I put one in, and my taillight has be working ever since!