There are settings in the service menu (the "hidden" menu) to adjust the reflected power sensitivity for HF, VHF, and UHF.? Perhaps those got misadjusted or corrupted (or perhaps the rig came from the factory misadjusted).? I don't remember which menu setting and I am at work so can't check my notes, but you can Google it.
Eric Forman KM4VR "Vinum lux Solis est, ab aqua coercita" - Galileo
When I say a good dummy load...I mean a Bird 8080, rated to 3.5 GHz.? Same symptom on another good dummy load and on an antenna known to be well matched.