Have you tried the pin method to pierce & probe the wires and measure the voltage right at the motor when this problem is occurring?
It looks like there’s no electronic control of the blower motor on your Corolla. Just the panel switch to turn it on and set the speed, a relay, and the resistor network. The ECM isn’t involved. There is a fuse in the circuit called the heater fuse, might be a good idea to take a look at that for signs of corrosion or burned terminals. Below is the nominal value for the resistors
pins resistance
1-2 1.5 ohm
1-3 0.5 ohm
1-4 3.25 ohm