I tried to design a 4 BIT ALU in proteus. The simulation first worked correctly but later stopped. i have used two 74151s , i.e. 8:1 Multiplexers, Two are working , Two are not.
Help Me Please.

 

This is the correct project file.

 

I have issue with Proteus trying to run CMOS logic chips. It crashes and slows down quite a bit. I also dislike their library selection and ability to add. Just needed to get that off my chest.