headset vs speaker
Actually Ray, I think Alto has hit the nail on the head.
If you have the speaker selected, it will still give feedback if the headset is worn. You have to turn OFF the speaker before it works properly.
The fact that it is not every transmission would tend to indicate a feedback rather than a switching problem with a headset.
However, the headsets are getting a bit long in the tooth, maybe a techie type could give his thoughts?