Rogue Zuess tubes won't bias in Ultralinear mode

Rogue Magnom 99 pre-amp
Sony 90000ES SACD
Martin Logan Summits
Audioquest and Acoustic Zen interconnects and powercords.
I love this amp but you can only set the tube bias in triode mode and the settings stay put. I set the tubes at 40Mva. I do the same in ultralinear mode which is my favorite mode and the bias setting holds for about thirty seconds and then the meter fluctuates from 15Mva to 75Mva wildly. I sent the amp to Rogue and they said they couldn't duplicate the problem. Has anyone ever expierienced anything like this? I've swapped speakers and cables but had the same result. Any ideas?
It could be motor boating. When switched to ultralinear, are the bias fluctuations cyclical? Is it possible that you have low line voltage? Bad solder connection on the ultralinear tap. Bad feedback cap. Possible tube socket issue. Possible issue with the switch that switches from triode configuration to ultralinear. Possibly bad/intermittent output tubes. Hope that helps get you started.
Thanks, The line voltage appears fine but I'll investigate these other possibilities after the holidays. I don't want to put the amp down at this time of year. I appreciate your thoughts. Thanks.
I had an issue with a rogue 99 per that they could not reproduce sent to them twice and in the end they believed me and replaced some parts they thought would be causing the problem (despite their not seeing it) and it has been fine since ( more than 2 years). I suggest you work with Rogue they are great.
Thank you, the problem with shipping the Zuess back and forth is that it weighs over 200lbs without the packing and pallet. It cost me over $400.00 in freight alone to send it last time then their upgrade charges were on top of that. I agree that they have an awesome crew up there and they really tried to figure it out but couldn't duplicate it. They are a very nice bunch indeed. Next time I'm going with the mono blocks. I'm looking for ideas so I can try to have it looked at locally. I appreciate your response.
Next time I'm going with the mono blocks.
Exactly my feelings after selling a Krell FPB600 ... in the future, only mono < 150 lbs so I don't need freight service.