Or alternately, the carb float level could be too high.

When cold, the extra rich mixture is OK, as it is the same as when some choke is applied, but when warmed up the volume of fuel vapour is excessive & the engine chokes from overfueling.
Looking at the little window in the front of the fuel bowl will very quickly verify if this is a problem.
The description of the symptoms seems to fit.

If this is the problem then it is easy enough to fix by following the instructions in the workshop manual.

It never fails to amaze me whenever a problem occurs, so many people start clutching at straws rather than get back to basics.
A full & proper tune up exactly as per the book will rectify a great many faults & if not, will establish those things that are NOT causing the problem.

If we are going to maintain our cars at home then the absolute minimum requirement would be a workshop manual, a reasonable tool kit, a timing light, some spares [like fuel filter, plugs, points, rubber fuel & vacuum hose etc] If points are still being used, then a dwell angle meter is always a good idea. [Did I say 'WORKSHOP MANUAL? Very important tool that one.]
Another really good diagnostic & tune up tool is a large diameter vacuum gauge if you can source one.

Without these things it might be better to take it to a competent mechanic. [Make sure he has a workshop manual]