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?

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.