Hi Tony,
Check out the distributor. Start with the cap - the HT leads are clamped to the top of the cap and it is possible the spark is shorting across to the other spark plug lead. If there is any sign of a track across the surface, scrape it away and then apply a little Vaseline or silicon sealer. There could also be a short to earth on the inside of the cap. Worn bearings in the distributor may also be letting the cam 'float'. These are not so easy to deal with - it has a parallel shaft and skimming it will also leave the timing gear slack. Just about the only way of rectifying this would be to turn up a thin walled sleeve, then Araldite it onto the shaft (use proper Araldite - not the rapid rubbish) - when set, skim the O.D. concentric with the shaft, then bore out the bronze bushes in the body to suit. (Beware, there is a small seal above the top bush).
Regarding the clutch judder: check that the 6 bronze bushes that fit through the flywheel are free to slide - these have been known to get clogged with dust from the clutch plates - especially if using first generation asbestos free linings. (Later generation asbestos free linings are woven like the original Ferodo material, and don't give that problem). If one or more bushes are dragging, then the plate will lift lopsided.
Regards,
Eddie.