1999 Dodge Ram 5.9L 4X4 not enough fuel

1999 Ram 1500 Van 5.2 L conversion vanI had same issue. No accelleration. Engine loses power under load, (throttle under 30% goes ok,  then when +30% it wheezes loads down like it is a diesel and starts to misfire) The van top speed dropped to 60 if you were on flat ground, and babied it. #0-45 going up any decent grade. Power slowly kept dropping over time, the engine lost vacuum under load, I had code for plug misfire, and just had no power under load. Lost transmission shifting points (and it started to sound a little funny too)  and heater vent controls would not work correctly when engine was under load.  Changed air filter, which had light oil discoloration on it.  No change  
Checked fuel pressure at 50 psi on rail ok Replace plugs, wires, coil and cap and rotor. No change Noticed dimming of hood courtesy light under load (voltage checked ok). Took off air filter, and reveved engine to where it lost power, I could see slight oil mist (or exhaust - fuel mix?) blowing up out of intake butterfly when the engine was acting poorly under load.  I had no smells from cat convertor. There seemed to be a good flow of exhaust out the pipe. Still I suspected plugged catalitic convertor. I beat on it with a rubber deadblow mallet and a little gray dust came out the exhaust. Took the van to exhaust shop and installed a new cat convertor.(also had to replace rust siezed O2 sensor).  On removal of the convertor, I was able to pour out a good amount of gray sandy debris (ash?) that was plugging the old convertor.  Now my van runs like it should.  Universal convertor installed for $180, sensor from auto parts store $66 I hope this helps you because I played with this for some time trying to figure it out.

Check the fuel filter and the EGR valve