Hi Tom,
Really this is just an assumption and may not be accurate. Another cause may be a cold solder joint.Really do not think it is a electronic componet due to the fact that it is intermitted. You can go through the band switch and here all the relays "click". 24 to 29 mhz is on the same band relay. According to print.
These are the little minature relays (sealed). So no chance of taking apart to clean.
I thinking that maybe the relay has become heavely tarnished due to the lack of use on those bands.
Thoughts?