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The extension cables are a good idea and getting some parts from brighteners is good also.
In other posts you said that you have stuff from closed TV repair shops. You might find things like extension cables and test CRTs among the items. You could find brighteners there also. Maybe replacement yokes too.
I didn't mean to imply that a scope should be used for every troubleshooting problem. You can determine frequency with your scopes to within about 5% which is plenty good enough in cases like these.
The way that the flyback circuit works it can only produce the high voltage if the frequency is within a certain range. I've seen parts destroyed because the frequency is way off. The horizontal output circuit is one that handles a lot of power and parts are under a lot of stress even when working normally.
If the horizontal oscillator is off frequency it could be due to many things, bad resistor, capacitor, coil, tube, bad connections. You seem to have some evidence that the problem changes with temperature so when you test these components individually they might not show that they are bad. You said you replaced all the capacitors, did you use the right type in each location? The CRT itself doesn't take a lot of current so having it disconnected shouldn't effect chassis voltages very much. Things like the yoke and the focus coil (if your set has one) need to be connected.
Since I don't have the schematic, if you could scan the section containing the horizontal area and post it that would help.
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