For the most part my ABS system works fine. Once in a while when I hit the brakes on something slippery like Ice or compacted snow, the ABS light comes on and all four tires lock up. The ABS system will then not work until the truck is shut off and started again. This problem seems to be occurring more and more often, always 15mph or less and is very annoying when I expect it to work. Does anybody have any idea what is causing this?