[Elecraft] K3: very low power out with sub rx turned on

[Ed K1EP]

Here is a strange problem that seemed to be intermittent last night 
and now has disappeared.

Last night in the 160 contest, when I turned on the subrx, I lost 
transmit power.  (It had been fine up until then.)  I was operating 
at 100W with (or without) the ATU into an okay ~1.5:1 load.  When I 
pressed the SUB (or did it from software), the SWR would go up to 
~6:1 as read on an external meter and power out would be about 3 
watts.   Yes, an external meter said that the power out was about 3 
watts and the SWR was now high.  To me that indicated I wasn't 
transmitting on 160M anymore, although the K3 display still read 
1.8xxMHz.  Either I was suffering from lack of sleep or something was 
strange.  I only have one antenna, there is no switching.  I was on 
ANT 1 (or else the meter wouldn't work).  After going back and forth 
from the sub rx on and off, I power cycled the radio, still no 
change.  I just decided to not use the sub rx to get on with the 
contest.  A little later I tried it again and could not reproduce 
it.  It probably had something to do with the frequency I was on 
(high up in the band) or the lack of sleep.   Any clues?