[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