can you help

i have a 2001 chevy camaro it has 64000 miles and is a 6 cylinder i was driving on the freeway at 65 then it stared acting funny so i pulled into a gas station to let it cool down but after i started it it wouldnt go over 40 so i drove it back home after a couple days i started hopeing that i would be normal but then it wouldnt go over 25 a couple people said it sound like the tranny and others said it was the catalac converter. so can you help me figure out the problem so i dont get ripped off by one of these repair shops