A thermal problem helps in narrowing the culprit(s) down. Test the 9V zener on the regulator board and the one by the 536 ohm resistor on the bottom board near the long shaft of the mode switch with a VTVM/VOM. If bad, use a BZX85B9V1. The second zener is part of the vertical circuit input circuit. Better to check and confirm than assume.
If you have freeze spray, start at the horizontal outputs and move back. A transistor could be temperature sensitive. I have seen this before while the transistor will check good. A brief release of the cooler is all that is needed. It is best to change both in a push-pull circuit to keep the balance to specs. This is for a power part or signal part.
If the cooler does not work on the parts, check the voltages while working right and when it has reduced sweep length. A drop in the B+ supplies will also do this. A muffin fan will help keep the area you want cool long enough to do testing. A piece of paper, envelope, etc. will help with air direction to isolate areas/parts. I have done this to isolate a thermal problem.
In my 453/4/A scopes, I replace more resistors to increase wattage and replace the electrolytics and most to all small (1mfd axial tantalums with film). I find most of the small tantalums will have at least several ohms or higher, e.g. 10 ohms or higher ESR. The maim filter cans I restuff with larger values, e.g. 200mfd to 270mfd UCY. The 10mfd output filter for the unregulated higher voltage, I put in a larger capacitance. Enough times the original electroyltics were already starting to go bad to being dried out to <1mfd when measured before restuffing. At near 50 years old, they are due to fail. This is my personal preference due to what I have seen and expericed. You do what you want with your piece. Doing the main filters takes the maximum amount of time and work in the areas to service in these series. I do not mind doing it because I know what is in there is now is new.
I know you will be thrilled when you get your scope working right. Let us know what you find is causing the problem(s).