Audio Research D70 mark1 + schematic

P.S. I realize the post regulated bias spec of -30.5 is low, spec being -37, but it's the instability that really hurts.
Thanks again

Well, This is a pretty old thread now, but maybe I'll get lucky.
I have a D-70 I'm troubleshooting for a drop in the bias after about 4-5 min of warm up.
I've have it current limited on a variac and was initially elated when I was able to bring the screen voltage up to spec by adding some capacitance across a 530 uFd cap. 
This seemed to cure a slow cycling splutter sound coming through the outputs and a day later, I tried adjusting the bias.
I found at the higher voltage AR recommends for setting the bias, that, although the spluttering was gone, it began drawing current after about 4-5 min.
I found this was due to a drop in bias voltage, which I've been trying to find the cause of since.
I tried monitoring the pre-regulated bias supply while separately monitoring the post bias supply.
I found the pre-reg stable at -54.6 VDC (Spec is -57) and initially the regulated was 30.5, but began dropping after 4-5 min.
The zener in the circuit measured neither short, nor open.
I've heard the circuit board is a bit of a nightmare to work on, as it's very thick and the tracings lift easily, so I'm trying my best to determine a cause before just tearing into it.
Any ideas?
Thanks