Do you have any kind of hiss from the speaker at all? When you key up can you hear a click from the speaker itself? This one will make you cringe, take a metal screwdriver while the radio is live (RX mode) and very carefully touch pin 4 of the audio IC, do you hear anything?
These radios are pretty solid but they are old, it could really be just about any cap in the circuit or a bad connection on the CB/PA switch, and those relays have issues too.
If you were a local I would tell you to come on down, I love working on that chassis.
Jim