Cooper11

I'm really struggling with this van. When I first start the van, put it in drive and take off, it accelerates and shifts just fine. The very next time I step on the gas it just sterts to die and almost stall. If I shove the gas pedal to the floor, it will downshift and then seemingly accelerate fine with high RPMs. Once you reach the spped you want and try to maintain the speed, it starts to sputter and kick. If I accelerate extremely slowly, it will go fine until it reaches a speed where more push on the gas is needed and then the same happens. If I stop, turn off and restart, I can put it in drive and the first time I accelerate it is fine and then it starts all over again. Here is what I have replaced so far:
1. checked vacuum lines - no issues
2. replaced both TBI gaskets - no difference
3. replaced EGR - no difference
4. replaced PCV - no difference
5. replaced fuel filter - no difference

I plan on replacing the fuel pump this weekend

Any suggestions???