I had a problem once with my TZ where a coil lug with coil HV had wiggled sideways and was touching the bracket. This of course energized the plunger, which was attached to the diverter to the mini. As this was all metal, and attached, it energized the wire form ramp as well.
I would be working on the clock, part of me leaning on the grounded siderail, and when my hand grazed against any of these parts, I would feel the shock, nothing major, but for the longest time wondered what this was about. When I realised that the lug was turned slightly in the bracket, I repositioned it, and no more issues.
Long story short, whatever part it is that is causing he shock, look at everything possible metal that it touches, and I bet you will find a short to something.