Thanls britmog,

Sorry but my explanation of the stopping/starting was perhaps a little misleading, since the 0 revs on the tacho will always occur when the engine has stopped. However I was attempting to explain the precise error, where the tacho drops to 0 rpm (no power from coil), and then immediately comes back on, causing the abrupt re-start.

As an extra update for everyone, I have been researching the XR700 ignition module, and also giving myself a crash course in coils and ballast resistors over the last couple of days.

Based on research and the helpful suggestions above, and with fingers crossed, I decided the easiest option yesterday was to pick up a new ballast resistor and standard coil pack.

Last night I also checked the actual resistances and voltages, and found the following :
> ballast resistor showing 2.4 ohms (should be around 1.6 ohms) NOT OK
> coil secondary showing around 13000 ohms OK
> coil primary more difficult to detect and less determined NOT OK

Based on these results last night I installed the new ballast resistor and the new coil, and then measured the following :
> ballast resistor 1.6 ohms
> primary coil 1.1 ohms
> secondary coil 13600 ohms
> voltage behind ballast resistor 12 V
> voltage past ballast resistor 6-7 V

These figures appreared to be in accordance with the recommended tolerances, and therefore at around midnight (and with fingers crossed again) we decided to fire her up.

She showed charging at 14.4 V at the battery (previously only 13.3 V), and was also maintaining the above voltages at the coil.

We then went for a half hour spin around our local steeets, and she reached normal operating temperatures and 100kph/3000 rpm without missing a beat !!!

Based on the above I am thinking that the original engine stop/start fault was caused by an old ballast resistor and an old coil, causing unreliable voltages. I am not sure why she was still able to idle, but at least the fault seeems to be fixed for now.

Thank you once again to everyone for your assistance and knoweldge and suggestions, and happy motoring !!!

Regards, Barney