You need to turn down your vertical speed, or else your 757 is going to stall. As your altitude increases, air gets thinner. Because of this, you have less lift and therefore you cannot climb at such high rates. Once you get much above 8,000 or 10,000 feet, I wouldn't set your V/S to over 2,000 ft/m; try 1,500. Find a V/S at which your airspeed begins to rise, or at least stops falling.