I've got the same thing happening on my '98 with a 5 spd.
From I have gathered, there could be a couple things going on. First of, is the symptomatic fuel pump issue of the fule filter clogging. Seems this issue also creates some stumbling problems when making sharp turns with less than a 1/4 tank of gas.
Also tied into this is the affect (or lack there of) the O2 sensors have, particularly when they pass the 80k mile mark. Old sensors will send false signals to the PCM that have a tendenacy to make your XJ run richer. Fuel economy also takes a hit with this...
I personally am going to replace the fuel pump once the stumbling start getting worst. Right not I only experience the stumbling every so often. I'm going to replace the O2 sensors really soon, just for maintenance sakes, if nothing else.