Hello Jaguar forums,

This site has been a huge help to me throughout my endeavours, and I think its time for me to post my own story and questions.

I bought a 1986 XJ6 Vanden Plas in November of 2022. The owner informed me of some issues, the headlights didn’t work, the right gas tank either, although it was brand new along with the fuel changeover valve. Whatever, I’d fix these problems when I got it to my garage. Despite these issues he maintained it was drivable, and I did indeed drive it the 240 miles home with no issues (save for some scary moments with the failing brake booster/master cylinder). I and my more-mechanically experienced father set about working on the car. We replaced the:
-Valve Cover Gaskets (Old ones were nonexistent and replaced with RTV sealant. Terrifying, but at least the valves were in tremendous condition)
-Brake Booster
-Brake Master Cylinder
-Repaired/replaced headlight and highbeam wiring/relays
-Repaired/Replaced front left indicator bulb/wiring
-Right fuel tank filler and gasket (Was more RTV sealant)
-Power Steering Rack
-Custom Exhaust Headers (Had a bad experience with a slow fabricator (car sat for 4 months) after being unable to find any fair condition manifolds for sale. There’s still a medium exhaust leak where it connects with the stock system after the catalytic converter delete. There is also the appropriate bung for the O2 sensor.)

Along with other cosmetic changes like
-Wooden veneer replacement for the center console, (previous was cracked from 37 years of sun damage)
-Replaced Aftermarket stereo with newer aftermarket stereo connected to new dashcam/backup camera
-New speedometer/odometer unit (Old one was stuck at 120,000 miles)

After fitting the exhaust, we discovered an issue with the engine cutting out whenever it was shifted from park. We did some more work, taking apart and refitting the air intake system which we had been cleaning seemed to fix this problem for some reason. Then the engine would run poorly, then die.
We replaced:
-The ignition switch
-Coil
-Ignition Amplifier switch (the GM Module inside the amplifier, not the amplifier itself)
-ballast resistor + repaired wiring
-new distributor cap/rotor (discovered burn marks)
-spark plugs
-spark plug wires (chewed on)
-coolant temperature sensor
At some point through this, fuel started coming out of the tanks while the engine was running. We
-performed a successful fuel pressure test
-new fuel injectors
-fuel pressure regulator
-Also tested for vacuum leaks with a smoke test, didn’t find any.

At this point, after all of that and losing spark, I decided to take it to the local Jaguar specialist. They did regain spark, after determining the air gap in the distributor to be incorrect, and replacing the right return valve and rewiring the changeover valve fixed the fueling issue. However, the car still ran extremely rough, and the engine cuts out about 40 seconds or less after its jump. I remember hearing the mechanic say he advanced the timing to help it out. They offered to keep it and keep working to make it run properly. Their 150/hr labor rates and invoices as descriptive as a sentence per 5 hours, combined with lack of care to even put the trunk panels back together or include the air filter in the car when I take it back leads me to shy away from them. I’d like to have a day soon in which I lock myself in my garage and just methodically work through possible issues, but I don’t really have many ideas. I’ve looked into ground straps, and learned of their importance. Upon touching the ground strap between the motor and body on the passenger side underneath the car, cubic centimeters of dirt fell off. I know it can’t help, and its certainly something I’ll clean up, but I don’t think that’s my issue. If it's dirty now it was just as dirty 9 months ago when I drove it 240 miles with no issues. Beyond that, I’m pretty lost on possibilities.

I can’t afford to let other people work on my car. I only went there for their expertise, but I believe this site has more collective knowledge to offer. I imagine I’m one of the younger members here, and my lack of background knowledge has been slowing me down the whole time. No one in my life understands why I dedicate so much time and effort to such an expensive car as a broke college student, but I’m certain people here do. I bought the car with the intention of persevering through any and all costs and efforts, and I intend to continue. I’ll post a video of the engine running/dying soon, any advice would be greatly appreciated.
Thank you