Here is a plot of the waveforms. Seems like your inductor should be OK.
Take a look at your waveform on a longer time scale to see if there are some pulses missing. Your "jitter" may be a clue.
If the inductor is saturating I would expect the triangle after L1 to look more like a square wave.
PS I didn't have the correct zener so the DC level may be off a little.
Wow! You just introduced me to LtSpice!
I tried to reproduce the circuit and , then noticed that I had the wrong resistor to polarize the zener (4,7K instead of 15K).
Guess it remained there after some tries and I forgot...
However by simulating the circuit with LtSpice I noticed that as the resistor value lowers, the frequency gets higher, and in fact with 15K i get the right , slower, frequencies.
I then tried it with my breadboard and the scope confirmed this behavior; also, with 300mA output the BC327 gets only lukewarm and doesn't smoke anymore.
Also, the scope shows the square wave is way more precise and "squared"!
By trying various values in LTSpice, I also understood the significance of the two capacitors that regulate the "on" and "off" periods.
I could even get the right operating frequency while keeping the "wrong" zener resistor (4,7K) and increasing by 5 fold both capacitors (22n and 4.7n).
I have to say LtSpice is a wonderful tool, I had never gotten around it since I had tried before its "cousins" (the various SPICE versions that are around), but never gotten to make them simulate anything useful.
LtSpice instead works like a charm!
Thank you ronv, you just opened a new world to me!
My best regards,
TheBlob