You say FD Runaway as if the FD controls the A/P. It does not. It reflects what the A/P is going to do to correct an error between the a/c performance now and what the MCP or FMC is commanding. It is perfectly possible to fly A/P ON & FD OFF. So if the a/c pitched up under A/P CMD which did not comply with the MCP selections I'd suggest the problem lies within the A/P and not the FD.
This is my understanding and I welcome to be corrected and learn more.