Hi there..
Although not a KWM-2, my old Heathkit SB-101 has done this a couple times in the past, and each time, it was the driver tube checking out, as mentioned by Peter above. In my case, that tube is being run pretty hard, and hot.
For me, the resolution was to grant the old man his well-deserved retirement and to buy myself a Yaesu FTdx-3000 as my own retirement gift. lol. 'Problem' solved.
Be well..
Chuck, N8NK