Have you checked the health and seal of your brake booster? X350’s were notorious for bouncing LTFT’s due to vacuum leaks in and associated with, the booster, and the booster design is all but the same on the earlier cars.
Does the camcover affect the fuel trims? I didn’t think it would.
As far as the brake booster goes, everything appear to be in good working order. I did replace the intake manifold cartridge for the brake booster a while back, but everything else is original. (NBC3184AA) It didn’t seem to make a difference to anything at the time I changed it.
Easy test on the booster, just pop the line off and plug it, check numbers.
The connection for the booster vacuum source, at the manifold, fails (cracks) and leaks, fuel injector seals can leak vacuum, the accordion hose at the throttle body shrinks into itself with age and can be a culprit, climate control vacuum, etc.
The fact that you’re getting a sporadic “0” that bounces, to me indicates a vacuum leak that opens up with heat.
You can also buy a fairly cheap smoke machine to vet out where leaks are.
As for the cam cover, it’s not an air tight seal / never was when new, and shouldn’t cause enough issue to effect trims. Make sure you’re using a late 4L gasket vs. an earlier 3.6 or 4.0, that fits into the grove and has the half-moons molded into the gasket vs. separate. If it’s holding oil leakage tight, it’s good.
What brand O2’s did you buy? I’ve had some weird emissions results after installing Denso units in X300’s. They like Bosch and NGK’s.
Last dribble - check your plugs first and foremost. What plugs are they, and how old? Plug ceramics delaminate from the base, almost always, with age. Sometimes the evidence is clear with a brown stain blown back up the plug, sometimes not.
Almost forgot, pull and plug the cruise control vac line, test.
The LTFT doesn’t really bounce. It just moves off of zero (or near zero) after a few months/several thousand miles of driving. I should start recording dates and miles. But it can stay proper for tens of thousands of miles, or just several thousands of miles, but its pretty consistent when it decides on where it wants to be, whether thats a 0, 10.2% or something in between.
I have replaced all injectors, accordion hose is in good shape, Bosch oxygen sensors on the car from about 20,000 miles ago. Plugs get changed every 2 years. NGK BKR5e, and I check the .035" gap. The corona stains are evidently normal, as a Jaguar TSB confirms.
Now the cruise control comment…my cruise control has never worked on this car. I’ve swapped out the cruise ecu in the trunk, successfully tested the dump valve and the vacuum pump seems to be working. The bellows also holds air. I gave up on it for while, but maybe there’s something here???
Definitely smoke test, the leak points are almost endless, from EGR to the air pump.
Is it tripping a CEL or are you just monitoring it repeatedly?