Narrow winding and odd number of layers indeed cause flux leakage and resulting spikes.
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your talkin about litz wire, it's best to use at high frequency because the plasma or electricity runs on the skin of the wire so the more strands the better, or use copper pipe for the area of skin.

http://www.dartmouth.edu/~sullivan/litzwire/skin.html

From electronic point of view the stranded wire behaves a little more like a Litz wire compared to a solid wire, because the conduction between the strands is not perfect due to naturally forming oxides and sulfates, which form a weak insulation layers between strands.  Also, strands are usually not ideally parallel and this limits their contact area and mutual conductivity.

Of course an older or previously heated wire will have more of this decreased conductivity between strands, making it act more and more like a Litz wire with time which has lower impedance to HF currents than a solid wire.

Now that is interesting, a PLL is just phase lock, and frequency off the PLL just tries to pull in with the VFO, so if you put a divider in series it will double the lock frequency, you need an AVC control circuit like an LM393 to track for maximum frequency (VFO)     against amplitude feed back circuit, hmm no one else has reported this problem yet. Any way nice video.

AG

Jeg,

what is your output of the MOSFET's?     Mine are outputting at the moment something like 70V squareish wave like 
My inductor coil is 11m long wire in 2 layers each 23 turns bifilar, so one layer say clockwise, then at the end back to the beginning, then again next layer clockwise.

The hissing sound points for me to a short or flashovers in a coil, but with only 3, 12 and 26 turns primaries / secondaries that sounds indeed strange.
Did you perhaps left the yoke old windings on there?

Regards Itsu