Rogue Zuess tubes won't bias in Ultralinear mode

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?

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.

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.