91 beretta and you are have the problem thhat it runs fine till you go up hill or at 35mph and above when you shift it sounds like it is backfiring and loss of power on hills?

Answer 1

It sounds like the inside of your catalytic converter has come apart, and as exhaust flow increases, it pushes the pieces up against the hole in the back of the converter and chokes off your exhaust. Limp the car to a muffler shop and have them test your cat converters, replace as necessary.