You don't have a heatsink. You just have a little piece of metal that might dissipate only 5W max. A real heatsink is extruded aluminum, is big and has many fins for a high surface area.
Maybe excessive heat has damaged the output transistors and T3.
How are you measuring the current in the output transistors? You must measure the voltage across the 0.34 ohm emitter resistors then use Ohm's Law to calculate the current.
Where did you get the antique parts for that old circuit? The output transistors are darlingtons. If you substituted ordinary transistors then the current will be very high.
Did you substitute any other parts?
Is it built on a pcb? A breadboard cannot correctly pass the high currents.
Maybe excessive heat has damaged the output transistors and T3.
How are you measuring the current in the output transistors? You must measure the voltage across the 0.34 ohm emitter resistors then use Ohm's Law to calculate the current.
Where did you get the antique parts for that old circuit? The output transistors are darlingtons. If you substituted ordinary transistors then the current will be very high.
Did you substitute any other parts?
Is it built on a pcb? A breadboard cannot correctly pass the high currents.