Why Can't I Stop Falling Asleep While Driving?

For the past six or seven years, I have had an increasingly difficult time staying awake and alert while driving.  I don't get the best night's sleep most of the time and am often tired, but the nodding off spells behind the wheel are unlike anything else -- and this is the only place this happens.  Initially, my vision stars to get blurred -- specifically, my eyes start to cross -- and then no matter WHAT I do, I can not seem to get focused.  I have blasted music, opened all the windows, sung out loud at the top of my lungs, slapped myself repeatedly in the face, bounced up and down in my seat, shaken my body and limbs... And it just comes back in SECONDS.  the only thing that seems to wake me up completely are near miss accidents (which, thankfully, have all been near miss, so far).  I drink coffee and diet coke to keep alert and this makes no difference at all.  Unfortunately, NOT driving is not an option.  What in the world is happening to me?

[Sallysmart]

Sounds like some form of narcolepsy, I would be looking into it but then again this should happen anywhere not just in a car. Have you pulled over to kick the tires when it happens?
I would be afraid to drive.
I had a similar problem when I drove a smaller scale street sweeper machine, it hummed and put me to sleep a few times. Because I was working mid shift I would pull over and hide in a quiet area and pass out for 10 mins and would feel much more alert after the snooze. I know it was the machine that did it because it had a hum and a low vibration with the brushes running, maybe that bit of vibration in the car is enough to hum you asleep, be careful anyway.