2005 Mack CV713, lightning Bolt

I have 2005 Mack CV713 with an 8LL trans, the lightning bolt symbol comes on at various times. It normally comes on when pulling hard or a major bump in the road. Also, it will go off on a bump in the road. When the light is on, I switch off the cruse and push the speed set button, if the the speedometer is still working I get 7-1 code. (service brake switch) If the speedometer is not working (which is most of the time) I get 4-1 code. (speed sensor) Now, while driving with the cruse on and set at any highway speed, the cruse shuts off when the lightning bolt comes on. I have replaced the speed sensor and pulled it after running for about a week the make sure it was still good. When the light is off the truck runs like it should. Any suggestions?