Clean all connections between radio, antenna and RF ground. Pay particular attention to the coax outside where it joins the backstay and goes through the hull.
Check that 12 volts is going to the radio, during a normal length transmission.
Inspect microphone wire and Pactor wiring
Check that the tuner is in fact tuning
If required, Improve the RF ground plane with more Cu foil, and make sure the dynaplate is clean.