[Elecraft] KPA500 Power Output

[N4CW at aol.com]

Sorry if this has been covered before, but I can't recall that it has. Is  
there any reason why power output
falls off over time? Here's what I did: using MMTTY, I "diddled" into a  
50-Ohm dummy load for one minute.
I initially had a power output of 525 Watts which went to 500 W shortly and 
 went down to about 465 W
when MMTTY "diddling" timed out. I repeated that process later, monitoring  
current and noted that it also
went down over time...makes sense! 
But my question is WHY? I was under the impression that I needed to run the 
 amp at 500 W on RTTY for
optimum efficiency because in an earlier posting I noted that at 150W  
output (in BPSK31), the amp was very inefficient and the fan was pretty loud. If 
 I can't derate the output and I can't run full output, what do I do with 
the  KPA500 in the digital modes?
 
Bert, N4CW