Amazon: Ignition again !!

[Bracpan]

I have been thinking of fitting a 123 dizzy for a while, but as i finally got the Amazon ( 1.8 ) running well again after resetting the points i did not see it as top of my list...until today.
Even if i don't use the car i always try to start it once a week and run it up and down the drive. Started fine today, drove it out of the garage as it warmed up choke fully off and it was running fine.
Then all of a sudden it went very very lumpy, it was ticking over but once i touched the throttle it felt terrible like firing on 2 or 3.
Drove it back in the garage, changed the the coil, checked all the leads, points gap and condition and connections, but no better, removed air filter and could see down the Zenith carb, no sign of flooding or leaks and fuel filter looks clean...stumped!
The all of a sudden it returned to normal. Revving clean and lovely tick over.
An hour later I decided to try it on the road again started fine, and it pulled away clean, did a run of about 25 miles and it felt perfect until I got about half a mile from the house and i had to pull over as the fault returned.
Kept the engine running, lifted the bonnet again did a visual check, pushed all the plug leads etc but still no better, then all of a sudden it returned back to normal, got back it and drove the last half mile up two steep hills with no problem at all.....I'm totally at a lose on where to start as again it sounds and runs fine....
Any ideas remembering this fault comes and goes very very quick...