F2B troubles (please help!)

[jstack]

Hello, I've been pulling my hair out over this silly F-2B project.
Here's a schematic and some project info http://www.moosapotamus.net/IDEAS/F2B/alembic.htm

The problem is the output volume is slowly falling after nearly a minute of being powered up.
When I probe around with a meter, I find that the voltages on the Anode are falling rapidly while the Grid and Cathode voltages are rising (the Grid fluctuates far more than the Cathode though).
Also slightly more strangely, the volume pot at power up has a normal taper with no noise or scratchyness, but as soon as the voltages start fluctuating ~1min it starts getting this prominent scratchyness and the taper seems to change greatly.

I have replaced; the filter cap from B+ to the anode, the whole Cathode circuit path (resistors, bypass capacitors), the volume pot, the tube (several times  )

These are the pin voltages
Low output: B+ 300 +/-3
1. 115v (rapidly falling)
2. 1.73v (rapidly rising)
3. 2.91v
4. 12.55v
5. 12.55v
6. 145v (rapidly falling)
7. 895mv (rapidly rising)
8. 2.42v
9. 0v

Normal output: B+ 300 +/-3
1. 180v (rapidly falling)
2. 80mv (rapidly rising)
3. 1.91v
4. 12.55v
5. 12.55v
6. 180v (rapidly falling)
7. 90mv (rapidly rising)
8. 1.71v
9. 0v

I have searched high and low for answers, I'm in way over my head 
Thanks for reading!