On rx , i have a low carrier , sometimes cannot hear someone vs another radio

I own a cobra gtl ; it was modified years ago. for other channels . so when i had it hooked up years ago , as i do now, same problem. its not the hook up or antenna. because when isubsutute w/ another radio ; the other radios recieves well .i know its in the radio. i can hear some one w/ a 4 carrier , noise level at 1-3 . where the carrier should be at a 9 noise level at 1-3 ?