Manual 5 speed transmission, car acts as if it.

Manual 5 speed transmission, car acts as if it is fuel starved, especially in 1st, 2nd & 3rd gears. At highway speeds the car throttle response is normal. Took the car to "Auto-Zone" & had diagnostic test done with no faults found. I've been told that the catalytic converter may be clogged, but if were the case, wouldn't it affect the car even at highway speeds?