'Handling the Big Jets' (Davies) will give you the answer, if you can still find a copy in print.
First found in the early 50's, an area to be avoided.
A few early heavy jet transports (707, DC-8's) found the problem with engines (Pratt&Whitney JT4A) that would get 'em to high altitude, only to find that the maneuvering margin was very low.
A bad combination.