When I used to repair Televisions and amps with valve powered sound stages, a thin film of oxidisation would sometimes build up on the pins of the valves.
The cure was usually to remove the valve, clean the pins with very fine emery cloth, use a fine needle file to clean the pin sockets, close the pin sockets up a bit tighter with the point of a needle file or tiny screwdriver, then spray valve pins and socket with a quick burst of WD40 or "Servisol".
If you do not have the tools, just spray the (cold !) valve pins and sockets with a moderate amount of WD40 or Servisol and shuffle the valve around in it's base. This usually cures the intermittent contact causing the problem.
Out of interest, I had a similar problem with one of the 12AX7s in my Peavey Classic 20 and a quick spray of WD40 on the pins and a 'shuffle' in the socket cured it.
This problem I have more often found in the pre-amp stages, as the fatter pins of the 6L6 or EL34 type output valves were less prone to this problem. With an EL84 output stage, you still have the 'small pin problem' . In a Television or record player/radio it was usually a single ECL82.
The point Malicious Steve raised about warming the amp up first was very valid in that a hot valve pin usually expands enough to eventually tighten the contact between pin and holder.
While you are doing that, it is always handy to spray the volume and tone controls as well if you can get easy access.
Hope that helps.
