2002 Lexus LS 430 Headlightss Acting Sporadic

In November 2007, I was told that I needed to replace both of my headlights because one had gone dim. I had them replaced by a non-Lexus mechanic. Since that time, the headlights have been sporadic. Sometimes both will come on; sometimes only the left; sometimes only the right and on occasion sometimes neither will come on. If I quickly turn the ON/OFF switch off and on a few times, sometimes they will both come on. When I have taken it to the mechanic, the lights appear to come on correctly, so it is difficult for them to diagnose the problem. This has now been happening for 9 months and I finally got pulled over by the police because the left light did not come on. Have you heard of this? What should the mechanic check in order to fix the problem?