Pietro,
Does your display flash HSWR by any chance?? My 818 would do this from time to time.? I thought something was wrong with feedline, antenna or something else.? I examined everything.? As a last resort, I restored factory defaults and the problem went away.? It seems that software became corrupt due to a power connector and the outcome was the radio software misinterpreting the output power and SWR.? When the SWR becomes high (as sensed internally) the radio output is reduced.? Save your settings and memory (use CHIRP or some other software for memory) and reset to factory defaults.
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Al Skierkiewicz
WB9UVJ
Al WB9UVJ