[Username1]

Well, if nothing else is changing with this symptom then you should look at the
voltages of pins on the picture tube that can change and cause this.... Common to
all 3 guns, like focus, other grids where all three are common. Is HV stable...?
Source voltage to the 3 screen adjustment pots.... Just before the focus grids..

When I say monitor the focus voltage, I mean stick the meter on it till it goes out, and
then comes back into focus all by itself.... Even if you take 5 hours to let it cycle
through a few focus cycles.... Diagnose it before replacing stuff...

And during this time does anything else happen...? Screen width, height, sound...?
No snapping sound...? Good ground on the picture tube...?

How about B+ Boost 386v is it steady while this happens....? Put the meter on it and
watch it while it cuts out.... then comes back....

I'm just guessing what chassis this is.... ctc12....? anyway, same ideas, different chassis..

If you got chassis number post it, if there is a link to schematic post it, or do the
conversion to appropriate parts off the ctc12 schematic I found off google.

.