[Elecraft] [K2]: K2/100 Frequency Stability?

[Curt]

I built K2 s/n 06424 some time ago, then added KSB2 and finally converted it to 100W a few years ago.  Usually used it for CW only, but recently moved it to main operating position and started using it on 40M LSB mornings. To my chagrin, received comment few days ago that I was off frequency, and this morning confirmed with another op that I was 50 Hz low.  The problem cleared after about 10 minutes. 

He mentioned that his K2/100 behaved the same way when first turned on and that this was known and "normal" behavior.

Is this true?  

Thanks and 73, Curt KB5JO