I have the same problem on mine, not quite as bad as Chris but noticeable enough. I won't use Powerspark as I bought a new electronic dizzy from them for my Pinto engined Capri and it was a bag of crap. The shaft was binding and could hardly be turned by hand. As for the arrogant individual who owns the company and who I had the misfortune to have to deal with, I'll leave my opinion of him to your imagination.
I ended up fitting an Accuspark ignition module to the original Capri dizzy and it's worked a treat ever since. I also disconnected the vacuum from the carb and set the ignition to around 15 deg advance on the advice of an old school mechanic who races old Fords and has a rolling road here in North Wales.
Anyway, back to Landrovers. I've also fitted the Accuspark module to the S3 dizzy, binned the points and condenser and replaced all the plugs, leads, cap and rotor arm along with a new set of NGKs. Coil is also new. Engine starts first time every time and ticks over like a Rolex but does have this annoying flat spot that goes away if you pull the choke out slightly. Carb has been overhauled as Chris did with his but not made a big difference. It's more noticeable on light throttle when driving such as in slow moving traffic and can be a right PITA as it kangaroos which along with the transmission backlash makes for an interesting ride .
I do have a bit of blue smoke on startup and assume this is due to worn valve stem seals but when running this clears quickly and cylinder compressions are generally good. I've put the flat spot down to an old and worn engine which will be rebuilt with a skimmed head to increase the compression slightly, new cam etc when the car gets it's restoration.