KX-T: kxtd 408 voltage spike on digital circuit

[Lancia Ersatzteilservice C.C.]

on my 408 I suddenly had a strange effect.
 
extension 201 suddenly started to make strange noises.
 
 measured and found pulsing voltage spikes about every 3 seconds instead of
the constant 14 V required. 
 
it looks like the unit wants to provide the signal, but then suddenly shuts
it down. I did measure the circuit, and could not find a a short.
 
Does anybody know about this effect ?
Thanks
Felix