2003 GMC Sierra transmission problems

Truck was entering major highway,traffic begins too slow due to rush hour ,seeing this occur I stepped on brakes expecting to slow down ,until I realized pedal had gone all the way to floor,then I let up an pushed to floor again-no brakes no room to go left or right impact was unavoidable,impacted car twice which pushed that car into another this truck was serviced by GM dealer for transmission cooling line and engine cooling lines monday tuesday and wednesday, no mention of brake lines issue, accident occurred on…

The transmission clunks when shifting from 2nd - 3 rd and at times the engine will rev and finally it will take off, this has caused an issue when going through an intersection. The dealership attempted four repairs and it is not fixed. The issue started when the vehicle had 6k miles on it and it is getting worse. The speedometer is sporadic in that it will show speed reduction/increase in segments - as if it were sticking. The erroneous speed indication causes an issue when going too slow or too fast in traffic.