*Assuming that you haven't seen a check engine light? If so, must retrieve codes for guidance.
*Good idea to check the coolant temperature sensor
*I would verify fuel pressure with a stone cold engine and see if it is in spec., which is 49 psi, plus or minus 5 psi. I have seen a fuel pump be marginal at initial start up after which pressure stabilizes after engine has run a bit.....
Good luck and keep us updated!
99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!