[Amps] Intermittent AL-811H SWR problem

[Jack Berry]

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?

Thanks & 73,
Jack - WE5ST