This happens as flybacks age, sometimes. With arcade flybacks, anyway. Cracks in the casing or insulation breakdown mean that as things warm up the focus swims a bit. In some cases the image will be stable, and always in the same place - so you leave the cabinet on for 5-10mins and adjust the focus for that point. Other times the flyback will settle on a different point each time, and there's nothing you can do but adjust the focus every time you turn the cab on, but after it warms up - assuming this is the problem. It sounds like it, but this a broadcast set so I can't say for sure.
Some flybacks become a fire hazard as this gets worse, be aware.