[Elecraft] Frequency drift

[Kevin Rock]

Yesterday after a QSO with a California amateur I received this in an email critique. 

<<<
Here is something you might want to check.  Your
frequency started to drift down after you cranked up
the power.  I think its happening with a lot of the
K2's with the amp.  By the end of the QSO I had
adjusted my rit down a few Hz.  My CW elmer has the
same K2/100 and it happens every time we go about 15
min in high power. 
>>>

Does anyone else ever get reports of their rigs drifting as described in this memo?  The amateur is mentioning when I turned the power up from 40 watts to 100 watts.  
Things were fine before that but the band decided it wanted to change and we needed to finish our QSO at a higher output power.

Is there a fix for this or should I just cool the radio with an extra fan?  (Or maybe some liquid nitrogen like we use when we overclock our computers :)
   Kevin.   KD5ONS