KM6YKX - Rob Newburn
I had a similar problem in the past. With my issue, it was an unsoldered/corroded connection at the feed point. SWR looked good until power was applied.
I think it has to be a bad connection/corrosion somewhere. Those antennas, the similar Tram, and even the Diamond are known to get moisture in the connections between the mast sections unless properly weather sealed.
I would try taking it down and cleaning and sealing each section connection and the feed point connection.
Good luck and 73,