[Elecraft] [K2] SSB Failure

[W4CU]

I bought a used K2/100, at the end of last year.  (Serial 5540 -  K2 MCU 
2.04P, K2 IOC 1.09, KSB2  1.08, KPA100  1.10).  I mostly use CW but do 
need SSB at times.  SSB had been working fine but recently RF output  
starts out fine - folks reply to my calls - but it then fades out to 
zero after about 2 minutes of use.   The same thing happens whether I'm 
running 100 watts or 5.  If I turn the rig off for ten minutes or so it 
will again show  full output but again fades quickly to zero.  I can 
still get full output on CW even after this happens.  If anyone has any 
thoughts on simple things I might check, please let me know.  Thanks - 
Tom W4CU