Irregular Rough Running

My 1973 198ci Slant 6 has an occasional rough idle. So much so that it stops running. However if shut off for an hour or so, it runs fine -- almost as smooth as new. The fuel system is OK (carb rebuilt recently), as are the emission controls. Has new vacuum hoses, spark plugs and wires. Most of the time it idles and runs well within a minute or two of starting. Sometimes it can't be kept running. Suggestions?