Chevy Suburban Engine Sevice Light

1999 Chevy Suburban...Whenever I climb a good sized hill or grade, the suburban looses power, bucks and the Engine Service light comes on flashing for a few seconds then stays on steady. It will stay on steady for a few hours, maybe a day or so, then suddenly extinguish. Took it to a dealer and they tuned up the car, changed the plugs and wires, etc. This however has not solved the problem. They suggest that it is from carbon build up in the valves. However, since then (and before) the car has not gotten better or worse. It only happens on hiils. (Load on the suburban). Could it be a bad Catalytic Converter?

Thanks,

Mike