Engine is missing at meduim throttle

Hello there. I could really use some tips on what to do next for my engine running problems. I have a 5.2 magnum in a 1992 ram 250 pickup. about 150,000 miles on it. It starts and idles exceptionally well and has full power when I floor it, but misses and jerks badly at any medium throttle position. So basically, I will not go any constant speed with out missing.
I have done: TPS sensor, fuel pump, fuel filter, spark plugs, spark plug wires, and complete distributor are all new and of decent quality. I cleaned the tank out which was ok looking to begin with. I ran sea foam and heet through the fuel system, and pulled the injectors and cleaned them (as much as can be done by hand). None of these things have solved the problem. I checked fuel pressure and it is at 31psi and when revved up it rises to 39. i did not check it with out the vaccum hose on regulator because I did not know to do that at the time. Im pretty much stumped right now, but will try whatever I can. Any advice? Thx
D

Have you rechecked plugs to see what they look like. recheck wires also did u replace rotor button with new distributor

You mean the cap and rotor? I replaced them as well, so the only ignition part that I didn't replace is the coil. I feel like a ignition system problem would be more likely to miss under heavy load, than at slower speeds on the road. It also lightly backfires while its missing. Anyhow I will check the plugs and get back to you. Thx.

It maybe the coil, is it possible your timing is off

i would almost be wondering if it is the crank position sensor. it is about the only thing you haven't replaced. have you tried pulling codes to see what the computer says?

did you ever figure out what the cause was?