1998 International 3600 Engine problems

We have a school bus that has had the transmission replaced four times so far and has had 8 new injectors put in it and it still doesn't run properly. The safety issue is it will not move when coming from a stop and when it finally moves, it is slow. Does not build up speed quickly. It is back in the shop and they cannot seem to figure it out. The first transmission replacement was put in the first six months of owning the vehicle, one in oct. 2003, One in oct. 2004 And one in june 2005. The vehicle has 102,000 miles on it and really should not have had to replace 8 injectors within that amount of miles. The bus has been down more than it has been on the road this school year.*Jb