Engine is hard starting

The engine doesn't start easily as it once did. This is an erratic condition. Typically when the engine is cold it will start much easier than once it's warmed up.
At a cold start up it will start almost immediately. After the engine has been running virtually for any period of time it takes more than fifteen seconds to start.
The fuel filter, spark plugs, plug wires, air filter, have all been replaced. IAC sensor was cleaned. This didn't effect the issue. Engine has 143,000 miles.