Hello Folks, I finally thought to ask some experts for their input to a problem that has plagued me for years. I'm not only old but combat disabled so go easy on me if I use the wrong terms.
The car in question happens to be a XJS Jag but I'm running basically a bone stock 1989 L98 out of a Vette. The 4 bolt main block and new 58cc alloy heads have less than 5 thousand miles. All emissions are in place and even the prom chip is stock.
The problem is as follows. Cold start up in any kind of weather the engine runs great and will do so even after it comes up to temp and goes into closed loop. Shut the car off and walk away for 1/2 hour and restart and it runs like crap, rough idle and the power is down. Walk away for 6 hours like yesterday at my Sons wedding and the restart was like it was after an overnight rest, ran great. No trouble codes are stored. Any suggestions?

 

Test for dropping fuel pressure when you shut the engine off.
Test for leaking injectors.

 

I will check the pressure on the hot restart, yet it does fire right up and no matter how far you drive when it's running bad it doesn't get better. The injectors are new from when I installed the new short block and heads. I was thinking some kind of vapor lock issue and even hit the fuel rails with freon to check but no change. The engine never runs hot and I installed dual Fans, one is controlled by a Delta Controller and ramps down in speed to cool off the engine after the engine is shut down. The other smaller fan runs only on a A/C demand.
The engine install is here http://crayonmedia.com/wayne/My_XJS/