strange spark?

I have a Briggs and Stratton 3.5hp vertical crank engine number 92902. This is a 1976 modal.
I have changed the points, spark plug, and tried a replacement coil of a known working engine (same make and model).
The main problem is this. When the plug cap is on the plug it will not run more than 20 seconds then cuts out and wont restart for 1/4 hour, then does the same.

BUT if i place HT lead without plug cap 1/2 inch away from plug tip it runs like new..