I had this problem a year ago and I thought it was the speed sensor.. It turned out it was the actual PCM. I had to get it replaced and I must say that cost about $2500 including labor and after hard work to find one..
I hope this is not the case for you, but just a heads up.
I must say thought that when that happened, I got so many errors on the information center some of which were traction control off, stability control off, variable effort steering off.... and the steering actually became alot "heavier."
Now this is happening again but without the variable effort steering part. Just the ABS and TC and stability control. I have to turn the car off and then back on. This time I am HOPING and PRAYING that its the sensor
