I was driving my 86 XJ6 with the 4.2L, and as I picked up speed to about 50 the engine started cutting out terribly. Then as I got to about 60 it would sound like its stumbling on maybe 2 cylinders before it just wouldnt really start again. I can start the car with the throttle wide open and it will run on maybe 2 or 3 cylinders barely before dying out. I had it sit for a while and was able to get it to run okay but after a few attempts it started doing the same thing. It will not idle though. I verified all my spark plugs are good and firing, and I dont believe the timing would randomly kick out. Im trying to figure out what it could be? I attatched a video below of what it js doing

 

Well, first, the scenario you describe is that of a fuel tank running dry. Did you try switching tanks?

 

Once it dies, try putting transmission in Drive and turning ignition to start and listen for fuel pump running in truck. Try 3-4 times holding 3-4 seconds each time. Transmission back to Park, If car starts and run a while, I would suspect Fuel filter.
Rgds
David

 

Fuel tanks are full, and I have run the fuel system off of a gas jug as well and it does the same thing. If I pop the return line off the fuel rail tons kf fuel pours out. Pulled out the injectors and verified they are firing

 

Car will not stay running long enough to do this unfortunately, but I will put a new fuel filter in anyway as I am sure it wont hurt. The car will not idle at all, only starts with throttle open