[Elecraft] K3 doesn't receive

[n0tt1 at juno.com]

Hi Gents,

My K3 has a problem and it's intermittent.  Maybe someone
has had the same issue and knows of a possible remedy.  
Here are the symptoms:
1.  Switch on radio.  Nothing being received except I can
     hear the neighbor's electric fencer and I can hear *some* "normal"
     powerline noise.  NO signals heard on the AM Broadcast Band
     which is very unusual.  It's like the signals that need to be 
     processed/demodulated aren't.
2.  Radio doesn't transmit when the no receive problem occurs.
3.  Turn off radio...
4.  Turn radio on and everything is normal, sometimes.
5.  It doesn't seem to matter if the radio is warmed up or not.
6.  Voltage to the radio is okay at all times.
7.  Audio out is okay at all times., i.e it isn't totally dead.
8.  Gently "banging" on the radio doesn't help...connections all
     okay in back of the radio.
9.  I can hear some noise increase/decrease when the RX pre-amp
     is switched in/out.
10.  The attached P3 sometimes won't update either when the freq
       is changed...recycle the K3's power switch and then all okay 
       with the P3....might be a separate issue.
11.  Other bands have the same "no receive" problem.

So...any ideas to help me zero-in on something to check first after
I open the covers?

Thanks,
Charlie, N0TT