[Elecraft] K3 Momentary no transmit

[N7FN at comcast.net]

I discovered a problem with my K3, s/n 83xx.
With nothing but power and a dummy load connected to the K3, while in TUNE 
on 10 meters, PWR adjusted to 15 watts, the K3 momentarily stops 
transmitting every 15 seconds.
When I push the TUNE button it starts transmitting.  After 15 seconds the K3 
momentarily stops transmitting for about a millisecond, again at 30 seconds, 
again at 45 seconds, and again at 60 seconds and so on.
The same result in both CW and DATA modes.
If I reduce the power to under 12 watts, the K3 transmits continually.
Has anyone experienced or duplicated this characteristic with their K3?

Thanks/73,
Frank
N7FN