Daniel,
This could be either a bad tube, or possibly a tube with dirty/oxidized pins. Take out the tubes and try to clean the pins. You may even want to get some very, very fine sandpaper (graded 250+) and lightly sand the pins. You can also try removing and reinserting the tubes in the sockets a few times. Sometimes just the friction of that process is enough to clean off whatever it might be that is causing the problem. Lastly, if you have it (or can get it), you might want to try using DeOxit on the pins. If none of this works, try using a different tube and see if that solves the problem.
Best regards,
Adam
