78 280z No spark from coil when hot.

A friend just bought an all original 78 Z, 80k on it. Ran fine the first couple of weeks, now when it gets up to normal temp. if you shut it off it will not start. No spark out of the coil. We swapped the coil, same result. Any advice on what to check next? He does have another 78 that we can swap parts with to trouble shoot. Thanks

check the wiring to the coil or even the resistance in the coil. Check it when its cold, and then when its warm. It shouldnt change much. If it does then odds are the coil is bad, but if it doesnt then you have a wiring problem.