Why is the engine hard to start while it is warm?

It is equiped with a 2.2L 4 cyl engine

While the engine is at idle it runs great, but after I turn it off and try to restart it, it is hard to start. It is almost like the battery is weak but I have had both battery and alternator tested and they passed. What would cause this to happen?

I have changed my head gasket, valve cover gasket, spark plugs, and the wires and it continues to do the same thing.