I might guess a thermal effect - perhaps either:
Control grid position or geometry changes, thus altering plate current, or
Connection problem with the filament, causing it to separate when hot but reconnect quickly as it cools, thus varying emission.
Perhaps you could watch the filament carefully to see if there is any visible brightness variation.
You didn't say if variation was fast or slow, but I am presuming slow. Interesting that so many should have that characteristic, though.
Control grid position or geometry changes, thus altering plate current, or
Connection problem with the filament, causing it to separate when hot but reconnect quickly as it cools, thus varying emission.
Perhaps you could watch the filament carefully to see if there is any visible brightness variation.
You didn't say if variation was fast or slow, but I am presuming slow. Interesting that so many should have that characteristic, though.