[Elecraft] K144XV on digital modes - disappearing power

[Kjeld Holm]

Dear Elecrafters, 
For a year or so I have had an internal K144XV transverter for 144 MHz in my
K3. I have not been using it very much - just a few FM QSOs. I have not seen
any problems. 
Now I have just started using K144XV on digital modes - especially FT8 (42%
TX and 58% RX). 
I started out selecting 10W output. After 4 minutes the output suddenly felt
to a few mill watt. 
Starting out with 5W or even 2W produced exactly the same result. 
But without regard to the starting point I could get normal output again
just by changing the output a little bit. It did not matter whether I
increased or decreased my power a bit.  
I did not hear any fans starting up and I could not feel any heating at any
sides of the K3. 
Can anyone explain what is going on - what I am doing wrong - what I can do
to keep the output stable? 
Vy 73 de OZ1CCM, Kjeld