no acceleration going uphill '05 Sorento EX

Sometimes when driving up steep grades, the vehicle will not accelerate, even with the gas pedal all the way to the floor. It doesn't occur 100% of the time, but happens quite often. My wife had been telling me about it for a while, but I thought it was just her driving. I finally experienced it myself recently while driving through the mountains of WV. The vehicle currently has around 63K on it. Any thoughts on what could be causing this? Thanks.

first thing that comes to mind,,,,partially plugged cat converter,,,just repaired mine on 02,,,,now,,lotsa power