You have the classic fault symptom, but it can be caused by many different problems.
The TV has a safety circuit which in those days was called a "Crowbar" which shutdown the power supply. Many things can trigger this circuit and include the following:-
- Power Supply over voltage on one of the lines.
- To much current being drawn from the power Supply
- Dirt in the EHT area causing insulation breakdown and sometimes flashover
The fact that it works for a short time after you have unplugged the TV tends to lead me to believe that a component is breaking down as it heats up. You will have to get a person experienced in these TV's to look at it so they can measure the voltages at the test points to discover the likely cause.