Chugs when driving for a few miles

I replaced transmission about 4,000 miles ago and was driving across country. Suddenly it began to chug and RPM's dropped. I made it to destination thankfully. I haven't needed to drive more than a couple of miles at a time with no problem until recently when I had to go about 25 miles. As I made it about 20 of my miles it began to do then same thing and still continues to do so if I go more than about 5 miles and drop below about 50 mph. Mechanic said it might be catalytic converter being plugged, No codes showing. Any help?