(Posts crossed-Snowman-al got in first while I was writing mine)
Sounds as though the UCL82 may be dodgy. When you say flickering, is that a bright light, like a spark?
The big resistor probably is the cathode resistor for the Pentode section. I would monitor the voltage across it with a 700 ohm anode load(2x1.5 k in parallel will be near enough) which will give you an indication of current flowing through the output valve. A quick ohms law calculation will give you the current which should be about 40mA.
You wouldn't want to burn out an output transformer too!
I would expect to see about 11 or 12 volts across it. The voltage should be fairly stable once the set warms up. It may well fluctuate when your flickering happens, indicating a bad UCL82
Info on UCL82( and the other valves in your set) at this excellent site.
http://r-type.org/exhib/aaa0477.htm