Sort of a long story folks, but I like closure.
I first came to this forum seeking advice for a transmission overhaul advice since I had just bought an xj with a dead ZF. Wound up going to a trans shop for the overhaul.
A while later I posted about a poorly running engine. I had never driven this car before buying but seller assured me it ran fine.
I'm a seasoned DIYer but unfamiliar with Jags. I've built engines, overhauled automatic transmissions and serviced most any part of a car. I'm limited by simple diagnostic tools so do a lot of it the hard & slow way, using my eyes, ears and head, testing individual components.
One of the symptoms my car exhibited was that it would run absolutely horrible, rough, buck violently and have little to no power UNTIL it could be coaxed up to ~3k RPM at which point it was as if somebody threw a switch, then full power was available. It once briefly broke the tires loose when it came alive.
A red herring thrown in the mix was an intermittant no start condition that was found to from the wire connector on the fuel pump melting. A new pump and connector cured that.
I spent many hours trying to diagnose and could find nothing wrong. I even went against my nature and threw parts at it in the form a used throttle assembly as I got it for about the same cost as diagnostics from the dealer would have cost.
An "expert"(Jag master tech)did a quick drive and was cocksure that the timing chain had slipped a tooth, even pointed to the bank where it had occured. Said "tech" was to do this job as side work at his home in the future, meanwhile I was to order parts.
Well, Mr. Jag Tech got greedy and wanted dealer shop rate and then some to change the tensioners so even though his diagnosis wasn't making sense I decided to do it myself in spite of dire warnings that without the Jag tools it couldn't be done properly. Members here had made me aware of the tensioner issues in this engine.
I had great difficulty understanding which cylinder is to be referenced for valve timing. Maybe I'm dense.
Finally getting informed about this I inspected and found valve timing to be correct but tensioners were originals(77k)and I had new style replacements on hand so replaced them. Not a hard job at all and done without any special tools.
Since cam timing was already correct this job had zero influence on running but gave peace of mind, especially after seeing the cracks in the tensioners removed.
This whole story unfolds over about a ten month period as illness and frustration gave good excuses to let it sit. Two days ago I drove past an independent shop with a lot full of Jags, antique to current models.
This stubborn old Swede finally raised the white flag and had the Jag towed over late that afternoon after discussing the car with a young mechanic who struck me as knowledgeable and trustworthy.
9:00 am yesterday my phone rings, car is fixed, one hour labor charge for diagnostics only.
Guess what it was. My trans shop had exchanged connectors for the O2 sensors so when the computer saw a rich or lean condition it told the wrong cylinder bank to compensate. If anyone is curious as to how bad this simple mistake will make your car run just swap them for a few minutes.
Trans. man was called and has offered to pay but wants me to call Monday, he was busy. we'll see.

So, moral of the story; It doesn't pay to struggle without proper diagnostic tools. I'd have never discovered this with my basic code scanner as I was unable to observe fuel trim. Both O2 sensors were putting out signals and every component I was able to test tested fine.
A sharp mechanic with proper tools found it in a few minutes. I could have been driving my car all summer.
For those of you in Chicago the shop was Import Specialists in Highland Park.
Many thanks to those here who offered help

Edit and update; Trans shop made good on promised reimbursement along with an apology.
Trans works great, I'm confident recommending AC transmission in Addison. Tony or Santo