1993grand

hi there! got a 1993 grand Cherokee 4.0L that has given me a headache. I have an issue where it will fire up fine when cold and idles ok, when I idle it up and get it warm or take for a quick drive then shut it off and try to restart, it takes a long crank time of about 10 seconds or more, I play with the throttle from full to none and that seems to help but still sputters and idles low once started then it catches on and seems ok. now the fuel pressure is around 38 psi to 40psi, unplug regulator vacuum line and it carries on up to 42 psi or so. I haven't checked the leak down rate. what do you guys suppose could be giving me this issue. also full rev and let off and rev again causes a harsh backfire that blew my muffler apart. any help would be great

1993grand

also I have installed a new maf sensor as well as an IAC and gave it a good throttle body clean, also new plugs, cap, rotor and wires. removed fuel rail and found junk in the rail, so I cleaned that and replaced fuel filter as well as installed injectors from an 88 mustang 5.0L, the issues had been before I did all this. the jeep was given to my wife and I as a project so now I am trying to figure it out.