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Since the problem is sensitive to vibration (and temperature) the root cause would have to be mechanical. Something is loose, could be a solder joint or inside a component like a tube, switch or resistor. Moving a switch may simply be generating the right type of vibration to effect another part. If it is a tube, you would need to recreate the same conditions during use of the tube tester in order to see the problem there.
Another example of why a tube tester can't find all tube problems.
Besides trying a new tube, clean switches and tube sockets, etc.
You may need to monitor some signal(s) during warm-up to track the problem down.
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