The final transmitter is out, this is normally caused by a bad antenna, you will have to take it to a shop unless you know how to fix it yourself. Before you hook it back to the antenna you have, check the wire with an ohm meter to make sure there is not a short. If you can also try and hook the antenna to a meter that checks reflective power, this is power that is reflected back towards the antenna because of metal, like the hood up. Too much of this and it will also burn out the transister in the CB. This and the mic are the #1 repair issue at shops.

It does sound like a power supply problem with low voltage because there is not enough current (Amperage) getting to the circuits in the radio to allow it to function properly. Try a different power supply or power cord or move it to another vehicle if mobile and see if the problem follows the radio if it does it could be a bad or cold solder joint in the radio or a bad power switch I have seen both often after these radios got some age on them. Hope this helps, if you have the expertise perhaps you can fix it if not take it to a reputable CB Shop and tell them the symptoms if they have not seen this problem offer the suggestions here.

Sounds rather like a power supply problem.