Charles & Sandra Cohen
+1 to Paul's suggestion:
. . . the contacts between faceplace and main body are often a cause of trouble,
. . . whether you're using an extension cable, or not.
Also, if the rig mis-behaves at high power, but is stable at low power --
. . . that often indicates "RF in the shack".
It would help to know what kind of antenna and tuner you're using,
. . and where they are, relative to the radio.
And, as well as a balun or ferrites on the antenna coax, a small ferrite (or several of them) on the mic cable might help.
. Charles / VA7CPC