johns wrote:

Please explain how a bad flyback can cause
the fault described by the original poster.

Picture size .. blooming .. is caused by high voltage changing.

Yes, but you havent established that the real reason
for the picture size changing is because the monitor
is getting confused by the obvious glitches on the
cable into changing the screen mode etc.

Generally when the solder joints start going,
it occurs across all the power handling connections,

Oh bull****. You only get bad joints where large components are
soldered onto the pcb and they start to fail due to thermal cycling.

and at the flyback.

Utterly mangled all over again. Flybacks fail quite differently.

Also, the electrolytics tend to crap out and
lose their capacitance .. even to dead shorts.

No evidence of that in the symptoms the OP mentioned.

I've been into lots of crts where all I did was
touch up solder connections and replace
electrolytics ... and the thing ran fine.

Sure, but that doesnt appear to be the OP's problem
given that its so sensitive to the video cable.

Of course that did not fix the gassy
crt which starts the picture to blur.

TRUTH: I've had people come in and say
"just make it work. I'll pay the cost. I like my
crt". So, I clean it ... resolder a bunch of
points .. replace horizontal output transistor
... replace flyback ... run tube through phosphor
restore and degauss ... set flyback focus ..
dark level ... remove and test all the big
electrolytics, and replace a bunch of them
... replace the the transistor amps and current
limit resistors on the crt board ... put it back
together and clean the case and screen to
nearly new ... and charge the lady 3 hours
labor at $65 per hour plus parts .. and hope
to heaven that the 90 day warranty holds :-)

Irrelevant to what the OP's symptoms indicate.