R3 and R2 make up a voltage divider, when the opto is on. In your circuit, the IGBT is starved for gate drive. It needs at least 9V of gate-emitter drive to fully turn on. Otherwise it is partially on and heating up, causing the weird effects you are seeing.
Voo is 9.7V, subtract opto VCE 0.2V and then voltage drops of R3, R1, R2 give only about 3.15V of gate drive. To fix that, I would delete R3, it looses 6.3V.
R1 does nothing unless you have high speed high current drive for the IGBT.
The circuit would not be suitable for PWM because the opto could turn on the IGBT somewhat fast but R2 means a slow turn off time, discharging the gate capacitance. The PC817 optocoupler is slow around 20usec rise or fall time.