[TonyP]

maybe the vert output tube has grid emission. measure the grid voltage over a period of a few minutes. does it rise? Is is already more than a volt or two positive? I think its supposed to be zero. If it is more than a volt positive, then turn the brightness down, remove the tube. measure the grid volts. if they dont rise, then its the tube. if the voltage keeps rising, could be the coupling cap is leaky. yes, new caps can be defective. most likely the tube had grid emission (aka tube is gassy).