JRT,

My previous problem was the motherboard. The rig is OK during the day because of regular use. The problem manifests itself when starting from cold in the morning.

I think Galland was probably referring to machine gun fire. The term 'cannon' was often used in earlier air warfare literature which in fact meant the humble .303/7.7mm machine gun or later the more powerful .50.
19 Sqdn used drum-fed 20mm cannon in the Spitfire Ib during the Battle but only briefly before reverting to the .303 Brownings.
Continuous stoppages due to the drum fouling the wing structure in g situations (and other factors) made the weapons virtually useless. The later MkVb had upper wing blisters to accomodate the upper part of the ammo drum which prevented contact with wing skinning. On the other hand, I suppose there was the possibility that Galland got in the way of a 19 Sqdn cannon armed Spit on an occasion when the pilot managed to get off a few rounds before jamming.
The severity of losses which caused the Luftwaffe withdrawal from concentrated daylight operations in September would have been arrived at several weeks earlier if cannon armed Spitfires and Hurricanes had been available and fully operational after Dunkirk.