I know this is a 5 year old post, but DT did you ever get it figured out?
I have a Cobra 29 LTD Classic from April 2003 sitting on my table, that I acquired with a stack load of radios. At first it TX'd and RX'd fine. Then today it decided to not RX at all.
When I went to TX, the needle moved backwards a little but no TX. At the time I had the radio plugged up to an external SWR/power meter and the meter did not show any dead key or modulation, albeit the TX/RX led changed from yellow-green to red.
It still works ok on the PA side. I even tried a few different mics but no dice.
Then I unplugged the radio and tried it again 10 minutes later, and it magically started working right again.
What gives?