[Amps] Intermittent AL-811H SWR problem

[k7fm]

I'm driving the 811H with <60 amps from a K3 into a SteppIR beam. I'm seeing 
high SWR (as much as 24:1) come and go.
Changed interconnecting coax, etc.
With the amp out of the lineup the SWR  is fine.
The reading I've done seems to say the tubes are failing. But I would think 
the problem would stay if that were the case.

Any ideas?

Do you mean less than 60 watts?  You say you are seeing a high SWR.  Where? 
Is it between the k3 and the amp or the amp and the SteppIR?  Hard to 
diagnose if we do not know the facts.

73, Colin  K7FM