It kinda sounds like you have a bad coil to be honest. Try swapping Left/Right flipper coils, same result or did the problem follow the coil? A coil testing fine with a regular multimeter doesn't necessarily mean it works properly under load, you need an inductance reading.

After it slowly died, did you test the replaced transistor?

Check wiring leading to coil to ensure there's no damage, might be adding resistance and starving it of power.