[Elecraft] KX3: slow build to full power on 15 meters only

[R Edward]

I have an odd issue that is only happening on 15 meters, and I notice it most in digital modes because they are 100% duty cycle. My first transmissions on the band are at reduced power from my power setting (usually 5 W), but the indicated output slowly builds over 2 or 3 transmission cycles to reach my selected power. The antenna is tuned beforehand with a 1:1 SWR, and my mic gain/ALC settings are all normal.

All other bands behave normally - the meter always immediately indicates full selected power. 

Any idea why might be happening only on 15 meters?

Thanks and 73,

Bob KH6BE