Hi Ed,
You haven't mentioned your antenna, however it could be like others have said and due to common-mode current getting into some of the control circuits.? Do you have a dummy load you can try out with?? That would be good as when it acts up again, you could switch in the dummy load and see how works then.
Another option, if you don't have a dummy load, would be to lower the power out a bit and see if that helps keep it from happening.
73 and have a good day,
Mark
