I have a suggestion, take your XJ to the dealer or one of those quick change lube places that has the transmission fluid transfusion machine.
I know it like $80 at Jiffy lube and my dealer charges $150 but you may have bad fluid in your transmission and adding new fluid not getting rid of the "bad".
I know a few people at work who have have this done and they report it "made it like new".
I had put in Mobil 1 ATF and my AW4 did not like it. I lost 3 mpg because of it. I had the transfusion done to get all the synthetic out of the transmission & torque converter. My dealer added an addative to the transmission then let it come to temp. After it was at temp then he hooked up the machine and did the fluid exchange.
My dealer mechanic is sold on the transfusion machine, especially the addative. He owns a mid 80s Ford F-150 he did it. He was ready to do a rebuild and thought he'd try the transfusion first. He told me "it saved me a rebuild".
He told me mine came out pretty clean. then a few months later I was in there just talking to the mechanic. He told me about another 88 XJ he did the transfusion on and the stuff came out dirty, he never seen it that dirty.
I went to the dealer since there is a 12 month/12,000 warranty. Figure if transmission problems came up, which they have not, I could get them to fix it.
Also in the late 80s there was a TSB telling to only used Dextron with the certification number printed on the bottle. It is a number that begins with "D". I hope the ATF you have been using does have the cert number on the bottle.
It sounds like you have been doing good maintenace. You could have a worn out torque converter. Do you ever tow in overdrive? I had a coworker trash his converter towing in OD.
HTH
Martin
