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you can try replacing parts OR you can try to figure out what is going on.
parts would be the resistors/pot/tubes in both the focus/HV regulation.
that's a lot of parts, and you risk taking a working set and making it not working from needless stress of part changing.
OR to figure out what is going on, use the HV prob and check the focus voltage from startup until focused. See if its changing and which way is it going (up or down). if no change in focus voltage yet change in focus on the screen then monitor the HV the same way, from startup blurry to focused and see if its changing. These are non invasive checks not likely to hurt anything.
after you find which voltage is changing effecting the focus, then you can diagnose the circuit.
I have an idea where the problem is but I prefer to withhold that to prevent unneeded part replacement. The voltage checks would confirm.
if you want to try one quick thing go for the obvious, focus problem, focus tube, that is what the tube jockeys did back in the day.
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