What is causing engine to idle ruff. Car has difficulty starting after warmed up

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Car has been idling rough especially when at operating temp. Recently car has trouble starting after being driven on highway for 30 miles. Car will not start without pressing on gas for a couple of seconds. Then car runs fine but idle is noticeably rough on idle. Car has 277k with recent tune up with replacement of distrubutor cap wires plugs fuel filter air cleaner. Throttle body was cleaned. Have about 3k on tune up.