[Elecraft] KS3 internal 100 watt amp (K3S/100) is intermittent

[k1dj at aol.com]

Rig:  K3S (100W with internal ATU), s/n 11064, P3, KPA500, KAT500, into SteppIR 3-el with 30/40 loop or wire dipoles/inverted-V's.

Problem:  My K3S 100 watt internal amplifier unit (K3S/100) has become intermittent.  

It first happened yesterday while I was running 100W on FT-8.  [I have done this hundreds of times, sometimes for lengthy periods, over the past 18 months with no problems.]
 Suddenly, my power output dropped to zero, and my received signals audio and P3 display also suddenly went quiet.   

After some fiddling around, I figured out that if I reduced the K3S power to 10-12 watts, both transmit and receive came on again — leading me to conclude that it must be my 100W amp unit that had dropped out.  

I rebooted the rig and the 100W internal amp unit kicked back on.  So I was hoping I had just triggered a protective circuit which had reset.   The rig worked fine at 100W for a subsequent hour-long multi-band CWOPs CWT CW session at 100W.  Then, however, a few minutes into the next CWT session hours later, the same thing happened again.  Power output suddenly dropped from 100W to 0, and received audio and a P-3 screen filled with many medium to strong CWOPs signals simultaneously went quiet.  

I reduced the power on the K3S down to 10W, and both transmit and receive worked again, but only at low power.
Please pass along any helpful suggestions, and thanks!
Richard Hoffman (Rich), K1DJScituate, MA