I also had problems with "Larger number of stages" during validation. I've made a faster short path finder, and solved the problem. But I still don't know if the problem was the time of execution, because the older program can run 3 times in the IDE test (calling 3 times the short path routine) without problem with time. So, the validation test is much longer or has a case that my old program cannot solve.
I think it would be a good idea, if validation tell us the cause of the problem (time of execution or wrong answer).