2000 Cougar

I have a 2000 Cougar. I have had the fuel pump(a few years ago) and alternator done.(last month) I am noticing the car is now acting as if it can barely make it up hills and some times like it is going to cut off when I am driving. Any ideas what it may be the mechanic says it is fine, but I can tell after driving it for six years it is not.

bad main cat and bad imrc are two things that come to mind.