Help! I need a good mechanic!

Just to give a different idea, back in the early 70s a friends shop had a then newer impala that had the same problem they did the carb, filter, fuel line clean and tune up and sent the car home, next day little old lady is back with same problem after checking every thing and coming up with nothing they did a valve job, sent car home, next day cars back with same problem out of desperation they replaced the 350 with new one and sent car home with granny, you guessed it next day same story.
They finally found the problem, remember this was a new car, a baffle in the muffler was lose and when it warmed up would block the tailpipe and the car wouldn't go over 25 mph