Low power on 2001 6.0

[rws1012]

I have a 2001 2500 HD with a 6.0 that loses power on hills and really loses power pulling a load. It has 148000 miles on it. Have changed plugs, wires, and cleaned mass air sensor after I got a code for it. Didn't seem to help. Drives down the road pretty good. Will cat converters stop up and cause this problem or is it something else? I did blow a large hole in the flowmaster muffler coming home the other day. All help appreciated