Loses power and spits then sputters and backfires when attempting to accelerate

Mine had the same problem as you describe "after driving it for about 10 miles it loses power and you can only go about 40 mph. If you try to accelerate it spits and sputters and backfires." I found burn marks on my computer board. I removed the computer board (found behind a panel on the passenger side, by the passengers feet, just ahead of the door) and sent this board to a company in Florida that rebuilds them. Also, a short wire harness cable had to be replaced. This cable is located under the intake manifold and connects the main harness to the fuel injectors. Rebuild of the computer board plus a new harness cost me less than $400.00. I have tools and did the work myself. The truck has run fine since.

I purchased a 1992 b2600i 4x4. The guy I bought it from said the ECM was bad. Sent it to Florida maybe the same people you used. My question is, my ECM was under the drivers seat is there another computer you said yours was on the passenger side?

I bought a Mazda that had the exact same issues I took it to a mechanic and he had to replace the distributor coil pack and finally he looked at the computer in the left panel it was completely burnt so he sent it to Florida and were waiting. So my advice is check your distributor.