I have the same problem, and just saw a neurologist. He said I have delayed sleep phase syndrome (I can almost never get to sleep before midnight, and usually much later than that), excessive daytime sleepiness, and hypersomnia. He wants me to try waking up every day at 10 am (yeah, right), and not take naps after 4pm, and only one nap of no more than 20 minutes a day. I probably won't take any, since they usually leave me more tired than I was before. I go back in six weeks.
Have you seen a sleep specialist?