Air molecules accummulate in the tube until there is a high voltage arc across the elements inside the tube. This captures the air molecules and afixes them to the internal parts and clears the condition. If the tube has been turned off for a length of time there may be enough air leaked inside the tube that it takes several discharges to clear it. This happens in all CRT's to some degree including TV's and monitors. When some TV's are turned on it sounds like somebody crushing cellophane with there hands the crackeling is so loud. This is the sound of internal arcing in the tube as leaked air molecules are captured. If your monitor is fuzzy and the sound of the arcing is loud you probably have the brightness turned up very high. Turning the brightness down a little improves the focus of any CRT. I thought everyone in electronics knew about this, at least anyone who has been to an electronics school. The leakage happens because it is not possible to get a perfect seal where the metal pins go through the glass neck to the electron guns inside. Also the lightest molecules such as hydrogen can migrate right through glass.
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