I had this issue when I first got my car, the only difference is my gauge cluster went nuts as well. I brought the car in and they said it was a firmware issue with the cluster and they had a flash/fix which didn't require a replacement cluster. It might have been a line of BS but whatever they did, it worked. The car has not shut off and lost power since. Here's my post from last year about it.
"When driving the car yesterday I looked down and my gauge cluster was totally dead with no tach, speedo, or anything. I had read here about this happening on some new cars I think. The messed up part is I figured it's an electrical problem so I turned on my headlights to see if those still worked, when I did the whole gauge cluster lit up like a x-mas tree with everything on for a second and then off again, then the tachometer started to work and the ABS light was on. At this point I didn't know if I should pull over and get it towed or keep going.. I drove for another mile or so just going off my rpm since I know what speed I'm going at 4k, all the sudden the bottom half of the cluster comes on and I can see my mileage again and the trip OD. At this point I figure it will be ok to finish the drive home, well I drive for another 10 miles or so with no speedometer when all the sudden i lose all power to the car and it felt like someone turned the key off, this lasted about 5 seconds and then pop, the speedometer starts working with all the other cluster operations but the ABS light was still on.I drove the car off the highway and shut it off for 20 mins, then restarted it and the cluster was working and the ABS light has now turned off"
2008 Persian Blue Exige S240/2006 Mazdaspeed6 GT