555 - Why are MOSFETs switching at a different speed than the one they are being driven at?

I used a 555 oscillator to drive a MOSFET array for my entry-level tesla coil setup. When I measure the frequency at the driver output (Location #2) everything seems normal. When I measure it at the MOSFET output (Location #3) I get an altogether different frequency. Why could that be? What improvements can I make to my circuit?

^{simulate this circuit – Schematic created using CircuitLab}

Here's the circuit.

Thank you,

Riccardo