I have no experience with that specific engine, but typically, the spark cable is not a separately available item.
You should access the spark cable and trace it carefully to its source, inspecting closely for sings that the cable has been shorting to ground because of a chafed spot on the cable insulation. If you find such a spot, you might be able to adjust the lay of the cable away from whatever has chafed it.
Mice seem to get their jollies eating electrical insulation off of various wiring so you might encounter that. Damage to high tension cables can sometimes be repaired by slavering the damaged spot with "brush on" vinyl liquid insulating sealant (available in home center electrical dept) or RTV (silicone sealant/caulk). Whether you use the vinyl compound or the RTV, you should coat the entire circumference of the cable for about 3/4 inch on both sides of the damage site. Let the repaired spot cure for at least a full days time before trying in service.
I will presume the "stop engine diode" to be similar to what Briggs uses. It is actually a pair of diodes.
The image below illustrates what the Briggs manual states about the Vanguard Twin diodes.

{{gwi:319398}}

Many thanks for your comments, the diode table is useful, certainly fits the bill and could be the cause, but I'm not sure if that sort of component would fail and then be ok for a while and then fail again etc. I would have thought when it fails it does so terminally, but as I said, sometimes the engine runs fine.
I will dismantle the covers and examine the spark cable tomorrow and see where I go from there.

The diode "per se" might not fail intermittently.......but it too is connected with some type of connecting terminals/plugs, and those would be subject to becoming loose, or chafed, or corroded............which MIGHT mimic some of the diode failure modes.

Thanks all for your help,

I checked the lead to the plug that was giving me trouble, but could find no evidence of arcing or wear.
I tested the diodes with a multimeter and they checked out fine.
So today I went and bought a new ignition coil for that side (which included the cable and connector as a 'sealed ' unit ). I have fitted it and have just completed cutting all 2 acres of my garden without a single 'splutter' which would have been impossible yesterday, so I am thinking and hoping that it has sorted the problem.
I hope by posting the result it may benefit someone else with a similar problem.

thanks again to those who responded :-))

Some very useful information there many thanks