Some thoughts:
Does a +8 of that era have an amplifier module on the side of the distributor? If so, they have been known to fail in this mode.
Swapping the coil is a good idea but a lot of the modern replacements are poor quality - if a new coil doesn't fix it, do swap back to the original coil.
Petrol starvation can produce very similar symptoms. A friend had this with an MG Midget and we were all convinced it was an ignition problem. It transpired that one of the fuel lines had started to break up internally and was causing an intermittent stop.
Just in case, here is a link to the instructions for your XR700 system - includes some useful information on fault finding:
XR700 Instructions
