As an aside before I start updating you on my ongoing

Richard —
As an aside before I start updating you on my ongoing travails with my 2003 Jaguar S-Type Sport (5-speed MANUAL) I wanted to mention that I will NOT be getting that XJR. The owner took it to somebody with a Jaguar-specific scan tool and that scan tool indicated low fuel pressure. The guy replaced the fuel filter and the car runs fine now. He has thus decided to keep it. (A generic OBDII scanner had indicated a problem with the EGR valve and I had mentioned how impressed I was with your previous answers on such a problem.)
Anyway, by way of background — which may or may not be important — I recently purchased this Jag with 151K miles on the clock. It was originally a lease car, sold at 2 years old and 24K miles to the family from which I purchased it.
The family from which I purchased it had it serviced at Hennessy Jaguar in Atlanta for many years before switching to independent garages. The family had $4K of recent receipts for service — $2.6K for timing chains, etc., and $1.4K for a clutch.
When inspecting the car I started it from dead cold. There was some vibration that led me to ask when the spark plugs were last changed. The owner did not know.
During the test drive, with the owner in the car, I did not drive aggressively. The car performed just fine.
On the drive home from Atlanta to Nashville the car had an overheating episode in a traffic jam. I pulled off the road quickly at an auto parts store, replaced the fan relay (which had clearly been replaced previously with an equivalent Ford item), let the car cool, and drove home without incident.
The following day, when driving around looking for an S-Type for my wife, the car had another overheating episode following a brief stop (heat soak, I believe). I let the car cool and drove home without incident.
I consulted with a Jaguar mechanic and he was confident that I had a bad fan module. I secured a used, updated replacement from Coventry West. When removing the old fan, it appeared that the failure may have been due to the “big” connector not being shoved fully home into its receptacle (creating concerns about the diligence of technicians who have been working on the car recently). I replaced the fan regardless and the new fan works fine. There have been no further overheating episodes and I do not think that any damage occurred during the two episodes I experienced.
Five-Hundred-Ish miles after all of this happened the P0171/P0174 codes, first one bank then both banks, began to appear, followed a few hundred miles thereafter by the codes and then “restricted performance” message (which disappears after two or three minutes).
I then proceeded to replace (as you may recall) the upper and lower intake manifold gaskets (which were the “yellow” versions), the IMT o-rings (which were the “green” versions but which I suspect of leaking regardless), the fitting on the passenger-side valve cover and tube, and the PVC valve, connector and pipe. I also replaced the spark plugs which were mis-matched (the driver bank had Bosch, the passenger bank incorrect NGK’s; one of the plugs was fairly white, the rest very light brown; I have pictures if that would be helpful). The replacements were Jaguar-boxed NGK from Coventry West ($16.50 each for pete’s sake!).
I did NOT replace the quick release fittings in the front and the back of the inlet manifold, the brake servo fitting and pipe, nor the connector at the front end of the PVC pipe (the one that connects to the air intake).
Following the repair, the car revved freer but was still a little rough (by comparison to the 2001 S-Type that I also own — I have no other recent experience with V-6’s). On my first 25-mile trip to the office, the error codes again appeared on the stretch of freeway where they had previously occurred (long, fairly steep incline).
I then did additional research, that evening cleaned the MAF with MAF cleaner (upon inspection with a jeweler’s loupe, the “back side” was covered in grey dust). I checked run-time data with a Jaguar-specific scan tool before and after the cleaning and the values after cleaning were slightly higher than before (before cleaning the low 4.27 g/s; after cleaning the low was 4.33 g/s, a negligible difference).
On my first 25-mile trip to the office the next day (which was coolish at 70ºF), the error codes did NOT appear on the stretch of freeway where they had previously occurred (long, fairly steep incline) but did occur on a similar stretch of road later in the day when the temperature was warmer (85ºF).
I then did some performance runs to try and identity symptoms. My method was not very scientific but here is what I observed:
— from a standstill in first, the car will rev freely but somewhat roughly (IMHO) to redline;
— in first from a rolling start there is significant hesitation and a pronounced power drop (like the ignition or fuel is being cut by the ECU — very rev-limiter like) occurring at 4,500 to 5,000 RPM wi