[Elecraft] KPA500: strange behaviour - any ideas?

[zendoc at netspace.net.au]

Hello Group,

I'm running KPA500 #1076 with firmware 1.23 installed.  It's
connected to my K3 via the elecraft AUX cable and is connected to my
PC via serial to USB adapter cable using FTDI chipset.  Interfacing
functions with the PC and K3 work flawlessly.

It runs perfectly apart from one intermittent peculiarity: 
unpredictably, when in QSO, the amp will switch from operate to
standby mode.  This does not trigger the Fault indicator and no fault
# is ever reported by the KPA500 to the utility program or to the K3
LCD.  It can't identify any triggers for these auto-tripping events.
I simply return the KPA500 to operate mode and continue the QSO
normally.  This occurs on average once per hour of operation, rarely
more frequently.  

Any ideas where to look for the cause?

Thanks and 73,

John
VK7JB