tube testing oddity

I have four 6550 tubes that are acting a bit crazy. When I first run an emission test the needle goes high into the " good zone " however after about 3 minutes the needle falls back down quite a bit. Still reading good, but not near as good as first registering. I tested other 6550's as well as EL34's and they all test normally. I have never had any tube do this.

Recommendations?

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