Strange Hard Start Issue

I have a 1990 XJ with 320K+ miles on it. I have owned it for 15 years and have taken good care of it. I am having a hard start issue that I have been chasing for six months. When the engine is ice cold it starts right up, but once the engine is hot and sits for more than 15 to 30 minutes it is very hard to start. The engine cranks right up when I leave for work in the morning. It also starts first try if it only sits several minutes when the engine is hot, like when I refuel. But once the hot/warm engine has sat for a while it is very hard to start, even when it has sat for like 10 hours while I am at work. I have replaced every sensor on the engine: MAP, MAT, O2, Coolant temp, knock, throttle position and crankshaft position. I have replaced the idle air control valve, EGR valve, fuel pump, fuel pressure regulator, fuel line to fuel rail O-rings, plugs, plug wires, distributor cap, rotor and ignition coil. I have removed and thoroughly cleaned the throttle body. I have checked all the fuel injectors and even replaced a few. I made sure all the exhaust manifold and intake manifold mounting screws are properly torqued. The cylinder compression and fuel pressure all check good. I do not see any telltale signs of a bad head gasket. I am at my wits end. I cannot figure out why the engine is so hard to start when it is hot/warm. Any suggestions????

Have you checked the fuel pressure long term when it sits and is warm. I am thinking you have vaporized fuel in the rail. You can try cycling the key on for two seconds, then off a couple of times before starting also and see what happens.

Thank you for the response - I have not done a long term fuel pressure check. I have tried cycling the key a few times before starting. That does seem to help a bit, but it still takes a while before the engine turns over. I will run rough for a few seconds then smooth out. If the issue is fuel vaporizing in the rail, how do I fix that? OEM fuel rails for that year are no longer available and I have not been able to find an after market fuel rail.

Since it runs rough after it starts, I am convinced it is heat causing the fuel to evaporate inside the rail. I suggest a cheap fix. Take some tin foil, wad it up and then flatten out, but leave the wrinkles in. cut in strips, whatever is convenient and wrap the rail and injectors with several layers. Also venting the hood will help too