Do you believe you are getting 8 hours of sleep every night? If you wake up several times that would mean you'd have to be in bed for more than 8 hours.
Sleepy when driving is terribly dangerous for you and those sharing the road with you. Please stop and get out of the car and get yourself back to fully awake before trying to drive again. There must be some techniques for staying away, but loud music never worked for me. I have to stop and stand up to fully wake up. For some reason I don't have that problem any more, it may be that I now pay more attention to driving - keep scanning the rear view mirror, keep a mental picture of where all the cars are around me, stuff that is in general good, and keeps my mind more active than just driving.
Hate to tell you this but I think you need to have a more complex, expensive sleep test, believe it's called a polysonography. I too was tested and treated for sleep apnea, but nothing improved. Finally my psychiatrist suggested getting tested for narcolepsy and she talked to my sleep doctor about it. It's basically like the first test you had but they continue into the next day and do "nap studies" to see what your brain does and how quickly you fall asleep. It took one of those studies for me to be diagnosed. There are treatments for narcolepsy. Let me know if you have any questions.
I sleep approximately 8 hours at night. I go sleep any time after 1:00 am and get up at about 9:30 am. Yet, I can easily go sleep for a further 2 hours during the day.
I've been having the exact same problem but im 31 weeks pregnant and my b12 is low so I've just started b12 injections, hopefully that will help me.
This makes me believe that further testing is essential.