It's the crank sensor!
Join Date: Nov 2007
Location: Central Minnesota
Welcome to the Jeep Forum......
I don't think this is a spark issue, and base timing is controlled by your computer; it is not adjustable.
First thing I would do if this were in my garage would be to hook up a fuel pressure gauge on the schrader valve on the fuel rail. Watch the fuel pressure and see if it remains consistent when the symptom occurs.
For a 99, you should see 49 psi, plus or minus 5 psi.
There certainly are other possibilities and this is a strange symptom, but consistent fuel delivery should be confirmed early in the troubleshooting process.
99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!