Radio going quiet in the afternoon

I would do a process of elimination. Since the radios receive goes quiet only in the afternoons, you can rule out coax or the antenna. These two items don't care what time of the day it is and then quit working.

Since you have another 29, just swap the old 29 for the newer, and see if the problem still exists. It the new radio receives fine all day long, then you know the problem is with your old radio.

It sounds to me like there's either a cold solder joint (which these newer Cobra CB's are getting famous for), or there's an electrolytic capacitor that's getting weak. I've even seen older cobra radios with cold solder joints.

 

Yes it can, especially when it comes to old electrolytic capacitors. Heat and age would be a capacitors worst enemy. Putting too much current through them and stressing them out, doesn't help either.
Heat may or may not make a difference when it comes to cold solder joints. The easiest test for a cold solder joint, is when you tap around on the radios pc board and the radio starts working normal again, then it's usually a cold solder joint.

 

Maybe you can answer this question for me. seems like lately you can't receive real well maybe there's too much trash in the air. I've got a Big striker and a 3600 Connex. I've changed antennas coax, swapped out radios and unless the person I'm talking to has a pretty substantial radio I just can't hear them all that well. I've had it looked at run SWR changed antennas still can't get it to listen well.

 

You bring up a good point about the knobs on the radio. Due to age and dust, these radio control pots and switches can get dirty over time. There's a lubricant spray called Dioxit 5 that a technician can spray the controls and switches. Sometimes cleaning these pots will help a radio work again.

 

Is the truck in your avatar the one your driving? Most trucks today don't act as a proper ground plane for the antenna system to work properly. The truck bodies are actually a poor conductor of RF. Are you familiar with bonding?

Bonding

If you don't have a proper antenna system, it won't matter what radio your using.