1999 jeep Cherokee runs and starts smooth. When put into any gear it drops hard and quits. But starts rite back up. What's the problem

If you have modified the wiring then I would put wiring back to how it should be

Does it change if you put it in neutral at a stop?

Check fuel pressure at injector rail.
Do It Yourself Diagnosis and Repair

check your Park/neutral position switch (automatic transmission), it may need cleaning or repacement
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it may need cleaning

Hi, i have a 1999 jeep grand cherokee, 4.0 litre, had the same problem, quits running out of the blue, driving down the road, or sitting idling. starts right back up or takes some cranking to get it running. found out that the crankshaft position sensor was going bad, cost 40$ for the part, but had to have mechinac replace it due to it's location, hard to get at. jeep runs great now.

another case of probable dirty throttle body. those 4.0 liters get real dirty in the throttle body. take off the air tube and spray carb cleaner down the bore and squirt that smaller hole the dtepper motor is in. get your tooth brush and open throttle by hand scrub away all that black goo in the bore and on the plat rinse with more spray untill sparkling. put together start. it will spuuter a bit cause of cleaner,

run scanner for trouble codes

bad distributors were common for this as well as o2 sensor

First check battery condition, second was the syc set after install?

have fuel pressure checked ...fuel pump maybe losing prime