When my car was still new enough to be under warranty I had an intermittant turn signal problem. The only way I was able to confirm that it was the bulb was to swap the two rear bulbs. The problem switched to the other side as well. A new bulb resolved the problem. The one that was not replaced finally acted the same way, at about 150,000 miles. In both of these cases, the symptom was that the turn signals would flash at double speed and someone that was following me said that he did not see the rear turn signals operate.
I also had a problem with intermittant no turn signals at all, first mostly with the left side and then eventually with both sides. A new switch solved that problem.
