A2 is a class A, but size 2 FSD.
Your previous was either a size 3 or 4 (dont remember), so even tho its a better class, its a lower size, not up to the task of heaving that ship around as well.
Thats what the "optimal mass" of the FSD means... how much it can handle. I bet your cobra is around 300, the FSD supports around 200... thus the lousy range.