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You really need a variac to troubleshoot this problem and there are several things that can be causing it.
1. Voltage regulator circuit in the power supply is defective. Typically, the B+ voltage in '80's and newer solid state sets is in the 120V to 130V range. If the voltage regulator circuit fails, B+ can rise to 150V-160V and this will also cause the HV to rise. When this happens, the HV shutdown circuit is triggered and this kills horizontal drive. With a variac, you can attempt to power up the set at 90-100V, while monitoring the B+ voltage. If the set stays on at 90V, gradually raise the AC input voltage and watch the B+ voltage. If the B+ voltage starts creeping up as you raise the AC input voltage, you have a voltage regulator problem. Voltage regulator issues are usually caused by the regulator IC itself, or any resistors or capacitors associated with it.
2. The power supply may be OK; but, there could be a fault in the horizontal output stage that is causing the HV to ramp up and trigger the shutdown circuit. The most common cause here is the capacitors that connect between the collector of the horizontal output transistor and ground. By the time these sets were made, these capacitors were usually either disc or film capacitors; but, I have seen them open and they can drift in value. They can also drift in value after the set warms up.
3. Loss of horizontal drive. This could be due to poor solder connections on the horizontal driver transformer (not the flyback - the little small transformer in the horizontal circuit) and/or the horizontal driver transistor. I recall working on one older Sears set that was doing what yours is doing and it was a bad horizontal driver transistor; but, the transistor didn't check bad with a meter. My theory is the transistor was good enough to allow the set to come up for a couple of seconds. Then, it would break down under load and kill the set. Also, any electrolytic capacitors in the drive circuit should be checked/replaced.
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