trouble starting on 1997 Dodge Ram 2500

My truck will start briefly and easly in the morning, run for about 20 seconds, then idle down and die. When I try to start it again, it just keeps turning over, almost acts like I ran it out of fuel, and I have to completely floor the gas pedal for about 20-30 secs before it will finally rev above 2000 rpm and not die (blows white smoke). I checked the fuel filter and it's not shiney white, but not overly dirty either. Also checked the fuel pressure btwn the lift pump and injection pump. The pressure gage needle is all over the place, up and down really fast. Lift pump replacement?? or is there something I'm missing- here?

I have a 98 dodge 2500 24 valve diesel and my throttle position sensor has failed three times in 240,000 miles and it was acting almost exactly like yours, once the sensor was changed it ran normal again. the sensors run about $350