Tube glows cherry red...then it doesn't ?

The other morning I warmed up my power amp and sat back with a cup of coffee to listen. A couple minutes later I got up and on the way out of the room noticed that one of the eight GE 6550A power tubes was glowing cherry red. I immediately turned down the bias and check the other tubes. They were all normal.

Over the course of the next twenty minutes the current drifted downwards and I kept rebiasing to keep the current constant at the nominal value. The setting of the bias control went back pretty much to where it had been before this mysterious event occurred.

For the past few days since it has behaved normally.

What gives?
1) Tube not plugged in firmly 2) Tube pins dirty 3) Tube socket holes are too large for smaller NOS tube pins 4) Bad pin soldier joint (usually on Russian tubes) 5) Tube is ready to s**t the bed. 6) Momentary household voltage change caused the grid bias to change and let the plate voltage "run away". Clean the pins/sockets with Caig DeOxit, then use G-X5 (high temp.) preservative. Caig also has some great cleaning swabs and brushes to fit the sockets. Next, look at the socket holes, and see if any look bigger than the others. You can use a very fine jewelers screwdriver to GENTLY close the metal socket contact. UNPLUG YOUR AMP FOR 24 HOURS BEFORE THESE STEPS, AND DO NOT GROUND YOURSELF TO ANY OTHER METAL ITEM (CHASSIS, STAND, ETC.) Definitely keep an eye on that tube! Also, is the GE 6550A power tube a proper tube for your amp (check with the manufacturer)?
I've had a few behave this way and go the next time I heavily exercised them. I'd recommend replacing it now.
I also recommend replacing the tube before it goes out and takes something else with it. A few years ago I had an output tube arc and not only blew a resistor but also some equipment upstream.
I would bet you have a pin not making good contact.Go to your local auto supply and purchase a spray can of electric contact cleaner.Spray your tube pins and sockets.Should take care of your problem. Mike
I would take the bottom cover off and take a look at the plate resistor and see if looks burnt or is cracked. I'll bet you had a high voltage condition in your neighborhood
and that tube or the associated resistor is weaker than the
other seven.
Get rid of it before it takes out a bias circuit or causes more damage. if a new one does the same thing, investigate other problems.
It sounds like the tube lost bias and went wide open. That is very hard on the tube.

I agree that it could be a bad connection between the socket and grid pin of the tube or somewhere further back in the bias circuits. FatParrot's ideas are good. I'm not sure about spraying cleaner in it unless it leaves no residue. There are many electronic cleaners on the market. Some leave a residue and some don't.