You can try looking through this thread, which covers a number of ideas which might lead to erratic running. There are other threads on the forum too, in fact so many you can take quite a while to go through them all!
http://club126uk.co.uk/forum/index.php?topic=9909.msg73620#msg73620A couple of immediate thoughts: fuel line too close to the engine, so when engine gets hot the fuel is partially vaporising *before* it enters the carburettor; secondly: is the coil breaking down when it gets hot?
Yes you changed the HT leads, but what about the coil itself?
The basic analysis is the same as in the thread I referenced - everything is fine when the engine is cold, it all stops when the engine gets hot. So what has changed? What is changing as the engine gets hotter? What other parts are getting hot too? And perhaps unexpectedly, like the fuel line?
Also, what is changing as it gets hotter? Is a split hose sealed when cold but leaking when hot?
Whatever is changing as it gets hot is reversing the change as it cools, but you say the time between failures is getting shorter. This suggests the failure item is getting worse. This suggests something like the coil, since every time the insulation breaks down it causes internal damage, so next time it fails sooner and easier.
Contrast with fuel evaporation in the fuel line. As the line position doesn't change, it simply gets hot then gets cool again, with no resultant change, why would the time taken to vaporise the fuel change? It would be the same every time you run the engine.
I recall someone once found the carburettor float to be punctured, and this led to similar problems. Although you fitted a new carb, was the *float* also new? It might be worth checking it out - even new floats can be faulty in manufacture, and I'd guess this one wasn't made all that recently...
