I had the same flickering problem with one of my MM 3W LED's. I cleaned the threads with a damp paper towel and then applied Pro-Gold to the contacts, as well as precision lubing the threads. It seems to have solved the problem-no more flickering.
If you don't have a Pro-Gold pen, get one. I don't know whats in it, but it really does work. Radio Shack had them 75% off a while back ( as well as De-Oxit), so it may be worth a look.