I had one of those, same year too.
There are several Jeep forums he could search to get ideas as to the problem. Mine did this same thing and I replaced the throttle position sensor and it was fine after that. There are many Odd Things Jeep electronics do (worth checking Merc forums too as the engine/electrics are all from Merc sprinter/Merc ML). Initially i thought it would be the main Engine management thingy but, as I say, throttle position sensor.
In the end I sold it (high miles) and bought a jeep grand cherokee 4.0 petrol, low miles and I only do about 3k a year. nice comfy cars with 3.5t towing ability.
Good luck