A while ago I had a 350 with an intermittent loss of spark. I found that the insulation on the wire that ran from the back of the magneto, across the back of the block, through a metal clip to the points, had hardened and cracked and was shorting out. As time was at a premium, rather than dismantling the mag to replace the whole length, I clipped the connector off at the points end, slid a length of heat shrink insulation over the entire exposed length and reconnected it to the points.