[Amps] What is wrong with my Alpha?

[Bill and Cindy]

Maybe someone here can figure this out. I can't, Alpha service can't.
 
I am the original owner of an Alpha 89. In March it started soft faulting a lot.
No red lights flashing to indicate "tune" or "swr". This usually happens on
15M but SOMETIMES on 20M or 40M. When the amp is tuned up with a
tone, there is no problem getting 1500++W out, no faults. Then when the
amp is keyed (say at 30 WPM CW) using either paddles or N1MM/laptop
it will fault at 800-1000 W, sometimes faulting at 400W after an hour of use.
 
I sent it to Alpha. They said it checks out fine, they were unable to reproduce
the problem. $150 service call for nothing. Now it costs $110/hr to even talk
to the service department.
 
I have tried 3 different XCVRS and 3 different 15M antennas and all possible
combinations. The grounding is good. Same results.
 
What does it mean when an Alpha faults but there are no flashing red lights
to point to the cause? Why would keying on CW cause a fault at 800W, but
the amp easily tunes up to 1500++W with a tone? The Alpha factory doesn't 
know......
 
Thanks in advance!
 
Bill, KO7AA in Tucson