[Elecraft] K2: KPA100 transmitted audio distortion 80 meters

[radiotelegrapher]

I have moderate distortion of transmitted SSB audio from my KPA100, but ONLY
on 80 meters.  The distortion seems worsen and improve from time to time,
with no apparent reason, and tapping components doesn't give a clue. 
Listening on a nearby receiver, and transmitting into a dummy load on 80
meters, the audio consistently sounds fine with the KPA100 off (at 10 W and
less), but is distorted with KPA100 on (at 11W and higher).  The distortion
is particularly noticeable at the start of a new syllable, which sound like
a loud crack.  A constant amplitude, like a tuning fork by the microphone,
sounds fine, so I suspect the TR switch or ALC.  My build is Rev 3, Jan. 18,
2007.  Searching the archives, I've found the usual causes, but the fixes
are already included on my build:  C31 is .22 mF, and R33 (1K) parallels
RC3. I tried changing R4 from 100K to about 39K, but it didn't help.  Any
ideas?  Dick, K0KK
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