I have been having problems with the light on my fan for awhile. The fan works fine. They both work from pull strings. The light would flicker and I would take out the bulb and put it back in and it would work. When it flickered again, I would play with the bulb until it stayed on. Now, no matter what I do, I can't get it to stay on, but it does flicker at certain points. How do I fix?