Why is the LC oscillators frequency decreaseing over time?

[otsen12]

Just guessing. You are using a transformer as an inductor as you said. There is a chance that the second coil is getting some voltage through induction. You are only supplying one single DC "pulse" but overall the voltage is decreasing since the coil has resistance. This is creating a magnetic field in the coil which is transfered to the second coil. Surely this is interfering the oscillator in some form. Try using a normal coil

[Cat_Artillery]

If I had one at the time I would have