cooling system has nothing to do with this problem
the symptoms lead to a clogged exhaust or a clogged carburator
also a bad fuel pump can do the same
if there are no backfires or the engine keeps running after the ignition is cut, I would say the ignition sustem is fine, but I'll look anyway under the distribuitor rotor for a frozen timing advance mechanism.
does it smoke black when the problem happens? this will lead you to an overly rich clue, like a choke plate that shuts or a bad needle valve.