[Elecraft] K3 sub receiver gradual noise level increase

[G4FNL]

Guys
I have a K3 that has developed what I think is a fault on the sub-receiver. It is S/N 883. MCU Firmware is at 4.86

If I switch on the K3 and enable the sub receiver, I hear equally as well as the main receiver but after a few seconds, an increasingly loud noise level builds up and the signal level drops down (in this situation, the noise remains present even when the aerial is disconnected). The main receiver operates without any such issues.

I  have saved a small video file showing the effect and how switching the sub receiver off and then immediately back on restores the receiver, until the noise returns and the signal drops out once again.  See 
https://dl.dropboxusercontent.com/u/43142183/2.wmv

I have checked internal TMP connections and also the multi-pin connectors on the sub receiver board, but having put things back together the situation remains unchanged.

Has anyone had a similar fault and if so, what was the solution? 

73 Graham G4FNL