I do not think a ethanol blend would cause any mis-firing. You need to do some more tracing & check codes to determine your issue.
Not probable but possible, ethanol bonds to water. Regular fuel (gas) and water may mix temporarily but don't bond, the water settles to the bottom. When the seasons change the fuel additives change, that water that settled to the bottom of the tank, either in your vehicle or at the station, can get sucked up into the fuel because of ethanol or additives and may foul the plugs.
The amount of water in your tank may surprise you. I once removed what I'd estimate as 3/4 of a gallon of ice from a 35 gallon fuel tank.
I've pulled, no start, fouled plugs out and tried to dry them with a cigarette lighter. Gas will catch on fire and/or evaporate fast, water not so much.
Change of season fuel causes a significant number of misses, stutters and hard starts most every year. It usually works itself out after the moisture gets moved through the system (either the station or your tank).
Fuel is not a blanket solution, but a possibility to consider before you buy a bunch of unnecessary parts.
From reading the posts it seems there are numerous threads about misses and stutters from the 96 model year showing up all at once. Other years also, but maybe the threads about the 96 just jump out at me because I have one. Not the first time a particular model year or series showed up with a sickness on numerous XJ's about the same time. I'm not discounting fuel as an issue, but it may be the same part or sensor is failing right about now on numerous XJ's. I tend to want to think out an issue and have a plan before I start swapping expensive parts out and/or start disconnecting stuff for testing. If it craps out completely it is usually a lot easier to troubleshoot than are the intermittent problems.
Codes can be a helpful diagnostic tool, they may also be absent for a bad part or present for a good part. I had one bad CPS on my 96 that was internally shorted and never did light a CEL or store a code. I eventually found it with an ohm meter and some luck. My confidence level on stored codes is around 50%, maybe it should be higher, but I remain skeptical.