Mazda BT-50: Transmisison problem

My Mazda BT50 started jumping out of fifth gear at about 54,000km, but Mazda and the local dealer don't want to know about it. There is an aftermarket kit made for the problem for about $520 plus $1200 to repair the gearbox, which indicates to me that it is a widespread problem and that Mazda should be shouldering some of the expense of the repair. Can you give me any guidance?