Now if we look at the H-bridge in the cole circuit we can see if one pair of transistor is being switched on the other one is switched off. Once you switch off the active pair it is possible that the voltage transitions / disturbance from the coil gets high enough in order to open the transistors that were NOT used in last cycle
(along parasitic collector diode).
Please observe in pic below how they add some diodes in order to protect form strange behavior.
I would recommend to add these diodes for test and obseve if they make any difference.
https://encrypted-tbn0.gstatic.com/i...CLfAEimYSTu1lw

Drawback might be they kill the "Bedini" spike that we might be after. If - yes - there are other cures in order to get transitors tamed.

So please check and find out if I'm wrong.

Warm regards
Johannes