I agree with Animera, set Pit Strategy to "none" to cure the problem.
Do not set your B-Spec drivers to pit at a particular strength level.
They do not regain enough strength while they are not driving, and then when it is their turn to drive again, their strength is so low that they pit the very next lap.
Rather leave them to pit only when they need fuel or tyres.
They will drive fine with no strength when they are a high level.