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300ci l6 misfiring and not reving past 1500rpm

I have a 1988 F-250 with a 4.9L 300ci l6 and ZF5. It has long tube headers, dual exhaust with a flow through cat and glass pack on each side. All of the emission equipment was removed except the O2 sensor and EGR pipe. I had to put a long block crate motor in it and it has between 3,000 and 4,000 miles on it. The truck has new sending units, all fuel pumps, all new filters everywhere, spark plugs, distributor, plug wires, battery, ECM, power steering pump, alternator, and water pump. I also cleaned all the grounds and replaced the ones with bad connections. The only thing that wasn't replaced that I can think of is the fuel injectors and intake manifold.

Before the new motor was installed, the truck ran rough and would load up with fuel when the motor was floating. As soon as I got on the gas or put it in overdrive it would be fine. A few months before the new motor every once in a while when I would start moving it would hit about 1500 rpm the stop revving like you turned the fuel off. Anyone with a truck with this drivetrain knows it is geared low so 1500 rpm in first gear is between 5 and 10 mph. I would shift and it would be fine again until I hit 1500 rpm. Again geared low so about 20 mph. Would shift again into 3rd gear and would be fine until 1500 rpm and the motor just stops. I would shift into 4th gear before I ever hit 40 mph. The same thing happened in 4th gear and 5th gear. It did this 3 or 4 times before the new motor.

I got the truck back with the new motor about 4 months ago and it ran like it did with the old motor. It still loaded up while floating the motor. About 2 weeks ago it did the thing where it didn't rev past 1500 rpm and I had to limp it home and it was fine until a few days ago. I was drove it up to our cabin about 4 hours away from home. It was fine on the ride up, driving about while I was there all weekend and the first 30 mins on the way home. It started doing it agin and I pulled over and turn the truck off for a few minutes and started again and nothing changed. I drove the whole way home with it like this because I wasn't paying for a tow truck.

The truck has been to multiple garages with the old motor and the new motor and no-one has been able to figure out the problem. I am taking back to the last garage it was at on Sunday and if anyone knows anything that could narrow it down or pin point the problem would be greatly appreciated.

Thanks in advance.

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