[71GMC_3/4T]

Check for spark the next time it won't start. If it has spark, check to see if it squirts fuel when you move the throttle.
When it does start, does it blow lots of black smoke, like it was flooded? If so, then your problem is likely ignition.

To check your coil, measure it while it's cold. Then throw it in the oven at 250 for an hour and measure it again. You want to measure the primary from screw terminal to screw terminal. Measure the secondary from the HV socket to the metal can. The resistance may change some, but if it goes open or shorted, then you have a junk coil.

I agree with changing over to HEI. Simple way to fix a lot of headaches.